Arcanists are specialized mages. They are optimized for casting spells that cause damage over time (DoT) or disadvantages (debuffs) to the victims. Most Arcanists use dark or fire-aligned crystals to cast their magic as they are the most 'offensive' elements.
Their abilities are often underestimated as the spells they cast are not huge and flashy, but the person at the other end of them eventually realizes their mistake when they've been rendered useless. Lowering an enemies' attack, defence and dexterity along with conjuring poisonous clouds are what Arcanists are known for. They are often the key to turning a battle in the favour of the Arcanist's side. Armies like to use them against heavily armored opponents. As with other mages, Arcanists are kept in the back of formations for safety.
Because their abilities are relatively niche and offensive, Arcanists don't see a lot of use outside of battle. They are extremely good at opening tight jar lids though. Apparently the grip strength of inanimate objects can be affected by their magic. Although it's quite a dark use of their powers, Arcanists are helpful in the agricultural industry as they can mass-kill stock relatively painlessly by lowering their defence and casting an AoE damaging spell. If they're skilled, they can even mass-poison pests on crops while avoiding damage to the plants.