re: What is Instagram for, really?

It is for getting attention, the purpose of all social media. For women, it is to showcase how physically attractive they are so they can get those cat-calls they so desperately crave (no matter what they say to the contrary). For men, it is attracting those attention whore women. It's also used by businesses, multi-level marketers, and entertainers for the exact same reasons. Whore for attention and then profit.