Firebase is a lot more than a database with a REST API. It pushes changes to you live, as they happen, in a way that is fast enough that you can use it for real-time gaming, collaborative whiteboards, and other apps could never work with a polling architecture.

It also does the work of managing distributed state of data across all of the clients and the server, and merging that data as needed to prevent conflicts in a distributed network like this. In fact, it allows you to run operations atomically across this distributed system -- operations that are latency-compensated, and work even in offline mode.

And finally, it can scale, with no work from the developer.