"Leadership" is another expression for behaviors that create hierarchies ... presumably for better food, women, and less fighting. Probably a sufficient explanation within Darwinian models.
Complexity theory hints at another one, probably more important. Hierarchies reduce task indecision. That is, one or two working together can make mutual decisions and still have considerable independent latitude.
Four or more people, relating equally, make few decisions ... there are too many constraints and too much time required to resolve them.
A hierarchy reduces the effective number of interacting units and the number of direct communication links. Decisions become possible again.
Please see my comments under "Complexity Theory" below.