The designated home console for an account gets all the benefits of Live/GPU without that account having to be signed in. It's not intuitive.
For example, Billy has Xbox 1 set as his home console, he can sign in to Xbox 2 and play with/against Sam who's playing on Xbox 1. If you have 2 kids each with an Xbox in their room, each should have their home console set as the one that is not on their room so they can share subscriptions and digital content. In this case, 1 GPU sub is all that's needed for both kids (unless you lose Internet access).