Well if we take a look at the human species back when we were still primitive a single male human could never defend their tribe/mate alone due to their physhical (spelling?) limit. So what did we humans do?
We invented tools (and we still do) to replace the lack of self made defence/attack power and we also prefer to attack/defend in groups thus making us stronger.
So basiclly, if one person is bad at, let's say ranged fighting (spears, stones, arrows etc.) he might be better at something else like melee combat while someone else is better at something else thus making teamwork the essence of our own survival.
And then there's also the cannon fodder role...
dunno if this actually helped or answered the question.
Oh look cute Tauren baby.