For one, artists and photographers use Instagram to promote their work. The viral nature of social media makes it a lot easier for posts to get traction compared to, say, having to go through making your own portfolio site and then having to do website SEO etc.

Maybe this doesn't bother the more technically minded, but as an artist you just want to get eyeballs on your art and any hurdle between you and that is taking time away from what you actually want to do.