Eyes are the easy ones. Everything follows once you admit that being able to sense light at all is better than not being able to sense it. Just a cell that tells you if light is on or off would help to get an animal to where the plants are photosynthesising and making food.

And that better eyes are subject to selection.. my uncorrected vision would probably lead to my deselection quite rapidly 'in the wild'. I wouldn't have had the chance to pass on my myopia to my daughter (the Boy seems to have skipped it, good for him).

But an important thing to remember is that evolution doesn't have a definite or preferred outcome- it didn't HAVE to be us, it's just that, having evolved the way we have, we can talk about it- perhaps the first organisms on this planet to be able to do so.