Web apps do not have read/write access to the file system. So let's say you wanted to write a text editor web app. You can't modify a user's local files so you are forced to store their data on your servers. You are forced to create some UI for integration with GitHub, Bitbucket, etc. You are forced to add a lot of stuff Sublime and TextMate don't have to worry about... you are forced to build an IDE.

This would improve a lot of that kind of thing.

The main problem it solves is to allow offline development in sandboxed environments like ChromeBooks and Windows RT devices (or even iPads if you had a BT keyboard).