[Pontiac]

"The question really is, 'Why did we go from having nobody on Earth with blue eyes 10,000 years ago to having 20 or 40 percent of Europeans having blue eyes now?" Hawks said. "This gene does something good for people. It makes them have more kids."

Simple it makes the person unusual in a non-crippling way. In other words attractive.

It is like when naturalist band a male crow. The male crow becomes a more attractive mate to the females. The male crow is not in any way different form any other healthy male crow but the band makes him different and in a way more attractive.

Blue eyes are the same sort of being different.

[G8 Diplomat]

Except that people, unlike animals, should chose their spouse based on individual qualities and the kind of person they are, not solely on how they look. Unfortunately many people do not do that.