A sample app for demonstration the use Android Bound Services.
Is an application component that can perform long-running operations in the background with no user interface.
These application have their own lifecycle independent of the activity or fragment that they were created in. So, services are unaffected by activity or fragment lifecycle events (onDestroy, onPause, and so on).
By default, services operate no the same thread that they're instantiated on.
You can use bound services when Client-Server interaction. The service acts as the server and someone component is a client. The client could be an activity a fragment a smart-watch or other application.
So, when you use consistent or frequent communication between some client and the service, in this case can be use Bound Services.
To learn more, click in the link above:
https://developer.android.com/guide/components/bound-services