Because it feels good to have the sun and the wind touching your skin!
I think in order to answer your question you need to get naked and spend a week in a nudist resort.
They want to be nakey with a bunch of other folks...I suppose.
No idea. :D
Because it is a great experience.
Going nude in a social setting is very pleasant and freeing. Also, to my experience, most people are much nicer and less pretentious when their genitals are in plain view. There's a lot more which I could say, but that get its, I think.
Couldn't find the right size in the swimming suit shop.
Lack of a proper wardrobe.
For indecisive people it's a no brainer: no pressure to pick out today's outfit! :P