If you're a developer, it may be hard to keep up with all the different Android form factors these days. Watches, cars, Google Cast, phones and tablets all need their own UI and code. So Google built a reference app to show you the ropes.
This app is definitely geared towards developers, so if you're a regular user, don't expect a fully-featured music player. However, it's designed to make use of all the different form factor features that Android uses, so you can get a handle on how to adapt your app to different devices. It's a handy way to get an edge on the competition.
A new reference app for multi-device applications | Android Developers via Android Police