Some cards allow you to get a Pokémon out of the Active Spot without spending Energy to retreat. This could be useful in a variety of scenarios:
Switching to a better setup Pokémon in the early game.
Getting vulnerable Pokémon out of the Active Spot after your opponent Pow/Reversals them.
Curing Special Conditions and other effects of attacks.
Denying KOs when your Active Pokémon is low on HP.
Cycling between two Pokémon that have Powers that can be used once per turn, but only when each is Active (e.g. Jirachi, Banette ex).
This switches out your Active Pokémon while also forcing your opponent to do the same.
Generally, forcing the opponent to switch is more valuable than doing nothing to them, as it can disable Cessation Crystal, or make your opponent spend resources to power up or switch out their new Active Pokémon.
This switches out your Active Pokémon. It also makes that Pokémon less vulnerable to a subsequent Pow, as you can use Warp Energy to pay for Retreat Cost.
Note that it has to be attached to the Active Pokémon in order to trigger its effect; attaching it to a Benched Pokémon does nothing other than provide Energy.
Warp Energy is notable for being an Energy, and hence safe from Stantler's Push Away, and reusable with bounce Energies.
These let your Pokémon retreat for free. Since they are Tools, they can be searched for with Castaway.
Fluffy Berry is generally superior, as it can be used multiple times. However, it has restrictions on what Pokémon can equip it, and those that can't will have to make do with Balloon Berry instead.
Note that these will not let you get out of a "can't retreat" effect like Pidgey's Corner, nor Paralysis or Sleep. Also, you can only retreat once per turn, and using these cards counts towards that limit, unlike other switching effects.
These function as switch cards when used on your Active Pokémon. They may also let you reuse when-played Poké-Powers, Evolution, Energy, and Tool cards.
Having thick Evolution lines with high-HP final Stage forms is a pull factor to using these, as they can survive more hits, and you can replay any Evolutions onto awaiting Basics.
Stages of Evolution reduces your Fire and Psychic Pokémon's Retreat Costs.
Played in Banette ex to cycle through Banettes and use Shady Move multiple times per turn.
Note that the Water-type Smoochum is preferred, as the Fire-type one can have Baby Evolution shut off by Lunatone.
This reduces both players' Psychic Pokémon's Retreat Costs.
Another option for Banette ex. Some Flygon ex δ builds use it too.
Extra Wing lets your Stage 2 ex's retreat for free.
A good counter to Stantler's Screechy Voice, if you can also deal with the Cessation Crystal it's likely carrying.
Played in some Kingdra or Dragonite ex δ builds.
There isn't much to this, it's just Warp Point without the opponent-switching effect.
Warp Point is more commonly seen, for reasons specified in its own section.
The Pokémon can't be trapped by forced switch cards like Pokémon Reversal, as it can simply retreat on your turn. It can still be KO bait, though.
After your opponent KOs your Active Pokémon with an attack, you do not have to commit to one specific Benched Pokémon taking its place right away. Send out the free retreater instead, and retreat to the most appropriate Benched Pokémon on your turn, after you have drawn a card and used Supporters to make a more informed decision.
If you use a switch card to cure Special Conditions from your Active Pokémon, you can promote the free retreater and immediately retreat back to your initial Active Pokémon, maintaining offensive momentum.
No Basic Pokémon has a sticker Retreat Cost of 0. Some of them, notably Latios ex δ, have Poké-Bodies that reduce their Retreat Costs to 0.
Pidgeot is a (or probably THE most) commonly played card with 0 Retreat.
Some cards are evil and have the opposite effect: they increase Pokémon's Retreat Costs. These are usually used in lock/control decks to trap a Pokémon that doesn't have enough Energy to attack or retreat in the Active Spot, while spreading damage to the Bench or recurring resources with Minun/Rocket's Wobbuffet/Raticate to not deck out first. Pow! Hand Extension can both drag a vulnerable Pokémon to the Active Spot, and remove Energy from it.
The most common of these are Team Aqua Hideout and Wobbuffet. Switch cards counter these, of course, but there are other ways of fighting them.
Team Aqua Hideout can be removed by playing Windstorm or your own Stadium, but your opponent isn't going to make this easy when they use Stantler's Push Away to get rid of those answers while they're in your hand.
Wobbuffet's Poké-Body can be negated by Cessation Crystal, or Latias δ's Dual Aura.