I guess sometimes it must be practical as well. In the summer heat especially in hot hot countries, you can't help looking hot hot.
I guess some people do it to be suggestive. Or well, maybe both.
I must say there is a female friend who looks cute in normal wear in the winter. I've seen pictures of her though when she was abroad in her tight fitting shorts, which I guess must have been for convenience, but well, I gotsta admit she looked even cuter!! Don't know how I would not be even more nervous round her like that!
I think it helps with body confidence as well. Preferably for guys after we've done sit-ups lol.
When is something revealing. I summer, you may wear a tank and short or bikini that shows off a lot of your body. But is that revealing? On the other hand, in winter you may wear a covering dress that shows a nice cleavage and part of your legs. Personnaly, I find that more revealing than an almost naked body.