Could you please explain when I'd want to use this vs. using the built-in Meteor mobile app system[1]?
This library is for building native Android apps (written in Java) that can talk to a Meteor server via their DDP protocol.
On the other hand, Meteor's built-in app system is for building wrappers around web applications that are built with HTML/CSS/JavaScript. For that, you don't need an Android client, of course, because it just uses their normal JavaScript libraries.