One of the reasons why Facebook bought Instagram was because Facebook noticed that more engagement was coming from pictures and other visual content. In fact, entire Facebook pages can be built with just pictures. You might have seen a bunch of political pictures of Hillary and Trump lately with a brief one-liner about the poster's view of them.
So in a sense, you're right, this is not something that hasn't ever been done before. But the reason why it exists is because at one point Instagram was actually a formidable threat to Facebook, thus the buy-out. It is much more for this reason than additional functionality or creative uses.
Another reason why it is still running strong is because of its viewership. They are mostly millennials and generation X users, which, unfortunately, have some of the lowest attention spans in history. Thus, the engagement on Instagram is useful to them because they can consume content on it quickly without thinking, which actually contributes to the problem of diminishing attention spans but is nonetheless the reason still.