Sometimes a bullet will land somewhere on the head (e.g. jaw or cheek) but miss the brain entirely. It doesn't make sense for someone to die to a jawshot which is why the brain should have its own hitbox on the character model.
NPCs that receive a non-brain headshot will be knocked down momentarily and suffer from a massive reduction in their shooting accuracy, spatial awareness and other debuffs that would logically happen when someone gets shot in the face