From the Kickstarter story:
The main use case this enables is developing offline in environments like ChromeBooks. 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.
Tim's plan is to develop the library in the open and license it under the MIT license. "It will be open sourced on GitHub as soon as the Kickstarter succeeds," says Tim. Our guess is you'll be able to fork it here github.com/creationix/js-git on Saturday Mar 30, 11:50am CDT when the Kickstarter funds.