Very good, you can go to the head of the class now. They are also trying to drum up a following for bigger and better things, that actually will put money in their pockets. On Instagram you can literally show off your breasts as long as the nips are covered, So for free a girl can advertise, show off her wears and then tell the avid followers that if they want to see more, go this site, which is obviously a pay site.

They are craving attention.

Of course,,,,,there is no other reason unless they're trying to sell bikinis. 

Yes, it is primarily done to show off their bodies. It is also done as an attempt to gain more followers (e.g. on Instagram), more likes, more attention (particularly from men) and more popularity overall. Some even do it to try and gain publicity on social media or even sponsorships. Essentially, it is done because these women measure their self-worth through the validation they receive from others on social media. It’s sad, it’s desperate and shows a complete lack of self-respect. By the way, I’m a woman.

Of course, why would anyone post their bodies if they don't want the public to see ? If they didn't want, they can just save it on their gallery -.-

What other reason would there be?

Edit - Of course it is vain. They want attention. 

Ya, I would think so.

Some, yes love to show off their bodies. But there are lots of girls that are very insecure about their looks and their bodies. 

Probably, if you've got it flaunt it so they say... I wish I had a bikini body, never ever been brave enough to wear one