I'm a front-end developer, and there's lots of service features to think about when building a single page app:

Should app data be generated from the server initially? Should the app support offline read-only mode? Should the app support realtime data? Should operations be batched and sent to a data source? Should the app be isomorphic?

And others. I built crudlet to help encapsulate services so that these sorta features can be added anytime without affecting the rest of the application.

That, and also to help decouple applications from a particular service. For instance, it'd be pretty easy to swap firebase with parse, or socket.io for pubnub if you want.