With a traditional IDE, you have to install it everywhere. With me, that would be a half-dozen computers, with 500MB lost on each one. With Cloud9, it's installed on the server and I can access it anywhere there's a net connection. Updates are centralized, you only need to do it once.

And it's funny, but the IDE is actually more performant in the browser. Well, I was using Aptana before, which is essentially Eclipse and that's known to be slow.

Also, it just feels natural to have your IDE right next to the end product. IDE in one tab, web app in the other.