I will answer this question in terms of AngularFire, Firebase's library for Angular.
Tl;dr: superpowers. :-)
AngularFire's three-way data binding. Angular binds the view and the $scope, i.e., what your users do in the view automagically updates in the local variables, and when your JavaScript updates a local variable the view automagically updates. With Firebase the cloud database also updates automagically. You don't need to write $http.get or $http.put requests, the data just updates.
Five-way data binding, and seven-way, nine-way, etc. I made a tic-tac-toe game using AngularFire. Two players can play together, with the two views updating the two $scopes and the cloud database. You could make a game with three or more players, all sharing one Firebase database.
AngularFire's OAuth2 library makes authorization easy with Facebook, GitHub, Google, Twitter, tokens, and passwords.
Double security. You can set up your Angular routes to require authorization, and set up rules in Firebase about who can read and write data.
There's no back end. You don't need to make a server with Node and Express. Running your own server can be a lot of work, require knowing about security, require that someone do something if the server goes down, etc.
Fast. If your server is in San Francisco and the client is in San Jose, fine. But for a client in Bangalore connecting to your server will be slower. Firebase is deployed around the world for fast connections everywhere.