I think the key extra feature is that you can share what you're working on with someone else and have both people collaborate in real-time (i.e. changes on eith

I think the key extra feature is that you can share what you're working on with someone else and have both people collaborate in real-time (i.e. changes on either end show up near-instantly on the other side). Supplemented by a voice channel, this could potentially be useful. JSFiddle et. al. don't have this, but I'm sure there are others out there that do.