My opinion is they like to wear them as undies because it projects the sexiness that I'm naked under my jeans, skirt, shorts or whatever. Personally I kind of like seeing fine panty lines when the clothing is tighter cause I find that sexy. As far as swimwear, even though I've only been to a couple beaches where they're allowed, I'd say it's to show off their bodies. Don't girls wear clothes for comfort anymore or to feel dressed with cute clothes that complement the body since those are some of the purposes of clothing or are they so body conscious that they think as close to looking naked is the best look for other girls or guys to see?
I wear thongs almost all the time, and not because I want to feel sexy. Regular (bikini)underwear has never really been comfortable on me, because it never stays where it should and I always get wedgies and it rides up a lot. When I'm hanging around at my house or a friends house I usually wear boy shorts or bikini underwear cause I don't have to worry about people seeing my fix my underwear constantly. So in public I'm almost always wearing thongs, because for me they are just way more comfortable.
*raises hand* I wear clothes for comfort, and pick out cute clothes that complement my body type. I don't mind thongs so much as underwear, although they're not my first choice, but there is NO WAY I would go out in a thong bathing suit. I'm just, ah, not that confident in my body. And besides, swimsuits like that are simply impractical. You can't really swim very well in a suit like that, and you certainly can't dive or go into the waves at the ocean without taking the risk of exposing yourself entirely.
But I think you have the right idea, that girls like to show off their bodies. There's the idea that guys will find it sexy if we show as much skin as humanly possible, but really, girls, do you want guys to think of you as just a sex object? Because if you attract them solely on the basis of showing off your goods, then it's gonna be harder for them to think of you in any other way. That's how I see it, anyway. And it's totally possible to look cute in a swimsuit that actually covers your bottom and your girly bits more effectively.