This necromancer spell allows you to deanimate certain magical minions. Minions that are deanimated are inert and can usually be carried around, and can almost always be reanimated into the original (or something similar to the original) creature.
Necromancers sometimes prefer to carry around deanimated minions instead of raw materials, as the setup time is usually shorter.
One danger of deanimate is that it is not always successful. Even the most skilled casters occasionally end up destroying minions instead of deanimating them. The exception are animated corpses, which simply fall down as their animating magic is dismissed.
Deanimate will not deanimate flesh beasts and clay men. For clay men, it's cheaper to simply form a new one, and there's no real way to deanimate a flesh beast without destroying it.