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.
Perves. If they weren't they would just stay home and be nude.
The extreme relaxation is worth it. Being naked only feels odd for the first five seconds. Everyone is polite and well behaved. The hardest part is getting dressed when it's time to leave.
Lack of a proper wardrobe.
For indecisive people it's a no brainer: no pressure to pick out today's outfit! :P