They're warm and comfortable. We like the coverage from the wind to. Plus it kinda reduces the amount of sand around our bottom. Some women like the coverage and don't feel as revealed. It's mostly for looks though; they're considered sexy and beachy.
i e a skin tight tiny speedo. or do you wear swim shorts, etc. what do you wear? a lot of guys wear swim trunks, do you ask them why they don't wear something more revealing since its more comfortable? no. because you don't want to see their bulge. your question has nothing to do with anything rational or practical. you just want to see ass.
Often it's just for looks, but it also makes for a happier girl in other ways, too, like the other answers describe. They may prefer to have some parts of themselves not on display as much, and that's perfectly fine. No one has any right to demand to see her body, and if she covers it up, good for her! Personally, I don't wear skimpy swimsuits because I think a guy should have to invest in me before he gets to see me like that.
to feel less revealing, hide imperfections, comfortable.. personally though I don’t like wearing like that and I also don’t like wearing see through cover-ups.. I just don’t like the feeling of “in between” and not really a fan of the look so I prefer something like this link and I only wear it if I go strolling around the island .