The ability to control music playing on a computer that is connected to speakers from a device that is not. This could also be integrated as a DJ system at parties to vote on songs to play. Otherwise, the basic feature is that you can see all signed in devices that are online and control the music that is playing on them.
As mentioned, this exists already and is called Spotify Connect (Premium feature).
If you need more info about it, please check those FAQs about Connect. 😉