The main selling points are that they are eye-pleasing and they can match theme of your outdoor area or something like that. If you like tiled in-ground pools, you can make your above-ground pool look liket that while you're swimming under water.
Main reason, I think, is to hind the wrinkles in the liner that must better.
I could see black liners being a really, really short fad -- you might have problems telling how far away the side or the bottom was and might find yourself bumping your head if you misjudged.
I hope you enjoy your pool! We have a membership at the pool in a subdivision about a mile away.