I think if you want to present the same identity on ferment you should use the same key. If you want to present differently, then you should use a different key. In patchworld, multiple identities is useful for like, parody or troll accounts, and there hasn't been much demand for that yet.

But for musicians, they might want to explore a different sound in a side project. They'll put that music out under a different name, because they don't want listeners to come at it with preconceptions of their other work!
Or, they want to see how their new music stands on it's own, without tying it to their already successful identity.