A few use cases:
1/ Make a modular backoffice for CMS based website. Make an app to manage users, an app to manage content, an app to upload and manage gallery of media etc.
2/ Make a distributed OS. All services can scale and use backend power beyond what a single node can provide. If you need to burst in the cloud due to a heavy process for instance.
3/ Could use it to provide a separation between the OS and the GUI, providing a UX I may like even if the backend server has to be Windows or Centos or ... Or remotely operating a part of my machine at home?
4/ What about reducing the cost of HW it needs to run? Chromebook style.
5/ What about being less tied in my Web App to Google Drive API + friends by adding a abstract VFS layer in between like a OS does? Same for texting, emailing? Sure it only uses part of this and don't need the client, but the architecture is well splited on this project between the two.
6/ It is multi-user as its core, so it can be shared across an organization and not only on one computer. You could even create one account per project/team and share a set of common tools / organization / folders that way across online services.
With the rise of web apps, it seems interesting to investigate the possibility of a web OS to administrate and make those ones collaborate more nicely. Or at least the possibility of separating the UX and the execution on two different nodes for general purpose systems as well.