At first glance, slavery doesn't seem it would be worth anyone's time, even if they want to make money off of someone else's labor. The moral and emotional problems are bad enough. When you add the time and effort it takes to obtain prisoners, make them work, and force them to stay put, the whole enterprise seems like a bad deal.
So then why does slavery remain a staple experience in D&D games? First, remember that the alignment system can be broad and encompass an almost maddening variety of views. With enough rationalization, people of many backgrounds and faiths can justify forced labor, at least for themselves.
If we want to look at profit margins, we can start with the 3.5 Player's Handbook reference to the 1 sp an unskilled laborer earns per day. If all slaves in a group are unskilled workers, then the 1 sp worth of poor fare they eat each day will balance things out. Add in the costs of capturing and transporting them and initially the slave owner is taking a loss. But prisoners can be worked beyond eight hours a day, perhaps for the rest of their lives, and cannot make further demands. They also cannot leave for a better deal, and some slaves are bound to have greater skills; just a few experts can bring in more cash.
But let's be honest: profit is not the only reason (or necessarily the foremost reason) that slavery continues. Most slave owning societies have many other motives for holding their captives, and the more needs that the prisoners seem to fulfill, the harder it is to give them up. So a Dungeon Master should choose at least three major reasons for a person or society to support slavery, and player characters would do well to figure them out, as well. They might not be good reasons, they might just be excuses or diversions or paltry justifications, but there are reasons even for the worst things that people do to each other. And since there are so many possible motivations to consider, they can be categorized according to the seven deadly sins.
Envy:
Although overlords would never admit it, many of them take slaves out of sheer envy of something they possess. After all, lives and cheap manual labor are not the only things that masters want, and there's so much more that a character can offer. In fact, the very reputation of a people can lead to their downfall. Dwarves have traditionally been admired for their abilities with metal and stonework – and they have been raided time and again by those who jealously want to exploit those skills. The longer life-spans and hearty constitutions that humans envy mean that dwarves can be worked to death longer than other hostages. (It also means that they cost more to purchase.)
But envy doesn't just drive people to attack other races. It also compels them to take what they can from neighbors just down the road, and it doesn't take much to summon the green-eyed monster. People who live next to natural resources are likely targets. Those who own beautiful land or possessions can be up for grabs, too. Sometimes downtrodden or mean-spirited neighbors will just be jealous that a village looks so damned happy, so warlords will attack and take slaves just to spite them.
Sometimes the jealousy of a god leads to slavery in the mortal realm. In many D&D worlds, the gods are very real and in communication with their worshipers - and if those who are given divine power don't obey, their abilities can be taken away from them. Deities also have their own personalities, histories, and rivalries, so clerics could be ordered to enslave worshipers of enemy gods, whether it's a part of the church's agenda or not. Not that all clerics mind, especially if they get to keep a few choice picks for themselves. But this could be an issue that causes clergy members to switch faiths or lose hope altogether.
Greed:
A thirst for coin and other trappings of wealth is only the beginning of greed. Inanimate objects can be wonderful and flashy, particularly in D&D, but they can't really react to their situation. (Unless they're sentient, and most objects aren't.) The desire to own sentient beings is an intoxicating reason to hold others against their will, because going against their will is the whole point. It's a declaration of ultimate power to have control over someone's life and death, and the thrill of that power might not wear off for a while. If it does wear thin, there are always new people left to conquer and more thinking, feeling “possessions” to control.
Some slavers work as kidnappers first, grabbing noteworthy targets and holding them for ransom. A few slavers have no real intention of setting up a fair trade and intend to keep their captive, and perhaps some of the people sent to make the exchange. Other kidnappers would rather get the gold and be done with further complications, so they aim to set a "reasonable" ransom that the target's friends and family can afford, at least with a little difficulty. Gauging the correct amount should involve Appraise and Gather Information checks, but if it is too much to ask, the target could end up in slavery permanently.
Greed-driven overlords can operate on extreme ends of the spectrum. On one hand, there's the mistress who is stretching her budget thin or who just wants to spend her money elsewhere. She pinches every penny she can by sending out poorly equipped raiding parties to grab the easiest targets. She spends pithy amounts on her prisoners' upkeep, relying on quantity rather than quality. Every captive is closely accounted for, and any escapee faces terrible punishment for making the mistress spend extra gold to hunt them down.
On the other hand, there's the luxurious magnate who uses chattel to highlight his wealth. He funds organized and trained hunting parties who only bring back the cream of the crop (after selling the lesser catches for top dollar along the way). His slaves are then garbed and trained to be the envy of guests and even other slaves, who think that captivity might be bearable surrounded by such finery. This kind of master has even forgiven the occasional escape – after letting it be known that freedom on the run is punishment enough.
Pride:
Some slave owners are thoroughly convinced of their own superiority (whether it's natural or unnatural in origin) and assume that it's their birthright to rule over others. While this drives some characters to become kings, not everyone has to aim so high; you can be king of your own castle and take slaves instead of serfs. This motive also leads some masters to believe that it is necessary for them to take control, since they know what's best and no one else is more fit. Many conquerors twist the idea of noblesse oblige to include the need to take care of “lessers” who cannot handle themselves.
Wealth and noble titles aren't the only things that can lead to delusions of superiority. After years of adventuring, a party could have become powerful enough to take over a town – and all of the people in it. Perhaps they raise their children to follow in their footsteps. Or a group of evil wizards might believe that those of good alignment are naturally weak, pathetic fools. When youth are registered with the wizards at the age of 16, they are tested for alignment and good youth are immediately enslaved. The prisoners are tested for alignment once a year until they reach the age of twenty-five; if they are evil for three years in a row, they are freed.
And when a mistress holds herself so far above her slaves, she might decide to keep them just for the fun of it – but it's really her pride that provides the laughs. The joke is on others for being too weak to defend themselves, and there's a perverse kind of joy that comes from making other people her puppets. They jump when the mistress says “jump,” just because she's the one saying it, and she loves everything about that kind of power. If things get dull, the mistress will just come up with new “games” to play with her toys. This point of view can lead to particularly cruel and capricious masters who treat people like disposable amusements.
Gluttony:
Life has a complex food chain and although slavery is a part of it, eating prisoners doesn't necessarily involve cannibalism (which is normally illegal within the empire). Some creatures, such as aberrations, are infamous for keeping “larders” full of sentient beings just waiting to be consumed, either entirely or in part. This is usually a terrifying and stressful experience for captives, since the fear of being eaten is common across alignments and races. Escapees might be particularly damaged or all but catatonic, depending on how much they saw and how people were chosen for the feast. Some heinous masters rely on the slaves to choose the day's victims, or set up games to settle the matter – making the prisoners feel partially to blame.
By comparison, hostages taken for sacrifice might seem better off, depending on the deity and clerics involved. Some evil clerics don't waste much time; they prepare corrupt services and kill the slaves quickly. Others torture captives for days before the ritual and summon dreaded outsiders to accept the offering. Some gods require sacrifices on a regular basis (on a holiday or a turn of the seasons) and with minimum or specific requirements. A deity might call for the murder of celestial creatures or one hundred human beings. Sacrifices that are demanded out of the blue tend to lead to frantic slave raids and sloppier tactics, since the god must be appeased – now.
But evil deities are not the only ones who require the sacrifice of lives. Neutral gods of the forest might require the ritual killing of any who harm the woods, and even a good god could ask their aged clerics to offer their blood for the harvest when they wish to die. For good and neutral faiths, sacrifice is a deeply holy occasion and a great honor to those who lose their lives. Even slaves that are captured for the ritual are not abused or upset any more than necessary. Some prisoners will eat and dress better than they have in their whole lives before they are slain. Good and neutral religions do not tend to ask for massive numbers of beings to be killed, either, preferring to rely on a minimum of bloodshed.
This can lead to some very interesting places in storytelling. If a race needs to consume sentient beings to live, is it evil for them to do so? Perhaps a neutral creature pays handsomely for a village's elderly, or for their worst prisoners. Is a dragon's maw somehow more evil than a death sentence carried out by an axe to the neck? And if a society knows that the gods demand lives to keep the crops growing and the sun burning, what else can they do but obey? A raiding party could be taking hostages from other lands to keep the clerics from choosing victims from the local populace.
Lust:
Sexual access is one of the more obvious reasons for keeping prisoners,
One of the more famous images of slavery is that of the harem, which gathers women to serve the sexual desires of one master. But there's no reason to restrict the harem to the same old arrangement; a woman in can keep pleasure slaves as easily as a man, and an overlord can give family and close friends access to the harem, if they wish. For the very wealthy, setting some captives aside just for sex ensures that they stay pretty and look pampered. From a certain point of view, it makes sense. For many slave owners, however, time is money and most vassals need to do more than have sex to pull their own weight.
Which leads us to a common reason for slavery: forced prostitution. For some pimps, it's not enough to manipulate sex workers until they hand over all of their money and are too scared to leave. Taking a person hostage and stranding them in a place where they have no ties makes it a lot more difficult for them to flee. Other times, offering prostitutes and services that other cultures have taboos about brings in more money. This does not mean that you have to go into dark, disturbing territory, though; taboos can be about races or sexual acts aided by magic.
And lest we forget, your game can reflect many different kinds of master/slave relationship. Lustful overlords do not always rule out kindness and regard. In a number of situations, pleasure captives are held in higher regard than most other vassals and almost as high as spouses. They are not commonly abused and can have great influence with their owners. Sometimes it's custom to refuse to sell harem slaves and to allow any children conceived with a citizen to become free citizens, as well. A few owners grant harem vassals freedom once a child is born with a member of the house, and some societies allow marriage between citizens and pleasure slaves.
Sloth:
Some slave owners would much rather leave the heavy lifting and dirty chores to someone that they do not have to treat fairly. Sure, they could pay some free person to do it, but why? Slaves often maintain fine houses and keep things running smoothly so that their masters can enjoy lives of leisure, and that leisure is important enough that the owners see it as their right – not a privilege that should be paid for.
Sloth is a strong motivation beneath some of the more decadent slave-faring societies, and it can definitely influence how they operate. Perhaps free men are paid for specialized labor, but lesser tasks (requiring less than 10 ranks in a skill) are unworthy of coin. Such masters might also pay little attention to what vassals are doing so long as their work is done on time and they are present for the morning roll call. After all, constant monitoring would require too much effort.
It doesn't take much effort to rationalize why one caste of society has to do the most work, or the worst kind. A creation myth can be turned against a group of people: since they were made last, they get to pick up the tab. Maybe clerics blame a local tribe for offending the God of Plenty and declare ownership of the tribesfolk for 50 generations to work off the insult. A band of refugees could end up enslaved just for showing up on the wrong doorstep looking for protection. Truly slothful slavers will make any excuses they need to obtain prisoners close to home.
Wrath:
Taking slaves to humiliate them in the long term is an ancient practice and was a potent fear in historical wars. Andromache, the wife of the Trojan hero Hector, had every reason to be afraid of her husband's death and even more reason to fear enslavement if her people lost. As the wife of Troy's greatest warrior, she was already a target for those who wanted to strike against her husband. When Troy fell, her young son with Hector was quickly killed; Andromache and Hector's brother Helenus were taken prisoner. They were an example of just how conquered the Trojan people had become, and they're an example of how slavery can be featured in your game.
Historical armies were also known to press people into service, whether they wanted to be involved or not. Anyone they could grab was put to work rowing in the galleys or fighting in whatever conflict was going on at the time. Prisoners might not be combat-ready but their lives were certainly expendable, and any extra labor would help ready all of the supplies that were needed. When battles were done, survivors might be given a choice between death or continuing to fight on behalf of their conquerors. Player characters could encounter raiding parties who are really out building a slave army – but for what end?
The notable thing about wrath-based slavery is that it doesn't tend to last as long as some other types. Someone can only be made an example of for so long before it loses its effectiveness, and the mortality rate of slave-soldiers is high, especially since citizens usually get first dibs on battlefield healing. Prisoners can also get away in the fray of a battlefield more easily than just about any other circumstance – although prized vassals like Andromache are known too well to just sneak away. If anything, hostages that are kept for humiliation are freed in disgust or sold for as high a price as they can fetch after the thrill of tormenting them is gone.