The usecase for this is not clear to me. Syncing is already done via the cloud, sharing as well. It's cool, but what's more to it?

Most users use it for adding push capabilities to their existing software. Eg you have a web app that uses a REST backend but you want updates to be forced into the browser instead of refreshing the page.

There are also those who use it for chat rooms, live analytics and event MMOs.

Real-time notifications, multiplayer games, status reports of some events happening continuosly without developing dedicated backend for it. These are the examples I could come up at the moment.