It's an convenient way to post about some trending topic, without creating a whole new community for something temporary. For example the eurovision sing festival, or some natural disaster that happened.
And on the other hand, it works for expressing some personal thoughts or memes without having to adhere to a specific topic. But with random strangers instead of only your facebook friends.
I think for these kind of needs, no other social media framework would comply better.
Also a reason why I avoid playing on easier difficulties