The idea is simple. Why create new features and options for connecting to a new device when its already developed?
Spotify can currently change the device playing music within a LAN in a way that is so seamless I jumped the first time when there were no delay. Why not use this already impressively implemented feature for chromecast? If chromecast was available as a device then it would make it a lot easier for us to use on the computer too, which is the main music player when anyone's home.
My guess is that a chromecast can't truly act as a device, but there should be a way to do this through some back-end trickery. Especially if there are unlicensed 3rd party programs that can cast (popcorntime for example).