The main thing it allows is developing offline en environments like chromebooks (like the new Pixel). I worked on Cloud9IDE for a year and it was a great experience as long as you were online with a fast connection. With this library, html5 apps will finally be able to do the full developer lifecycle. They can clone from github to the browser's local file storage when online, work offline using an editor like ace or codemirror, and then when they are online again, they can push their changes back to github. I'll implement branching, merging, diffing, and as many other awesome common tasks from git as possible.
I've wanted this for years and I'm super excited to finally be able to create it when this kickstarter finishes!