Well, there are social reasons of course; sexual selection and all that. And a good conspicuous, contrasting colour could help improve one's attractiveness by improving one's social signals.

Physiologically the only effect that I know of is protecting the functional tissues, such as the lens and ciliary muscles,  from bright or UV light.  The darkest eyes tend to be most strongly selected for in sunny climes, though this seems to be less strongly marked than having a fair skin in non-sunny, cold-temperate zones, and black or dark brown near the equator.

I f I had my way, humans would have a light coloured fur like a cat's, covering a black skin. But no one ever asks ME!!!

Grrmmpphh!