But most human variations are recent! We've changed more in the last 50,000 years than the previous 1M years. I'd say, instead of a real purpose, any 'breakages' are likely coincidental to some other, more advantageous adaptation. Getting smaller and more calorie-efficient (to suit the village/town/city/urban environment) may mean biochemical changes that change our mental state. Not because we 'need' that mental state, but because its better for the clan if more of us fit closer together.

So many adaptations are sledgehammers for a smaller problem. Evolution doesn't plan; it just diddles around until something (sort of) works. And our bodies are the unfortunate result!