Some features, such as melanin levels, are due to evolutionary pressures. High levels of melanin protect you from the sun, lower levels of melanin boost your vitamin D production.

Other features are just random. Over time, human populations were affected by random changes that weren’t exactly useful but not detremental either. They’re called sprandels. According to a quick Google search, blue eyes don’t seem to have any advantages, but not really any major disadvantages either, so that would be an example of that.