We had a boating moment the other night - one of our 8D batteries was going bad - lots of outgassing, high temps, bad scene overall. Ok - disconnected battery, temps etc came down. But how to isolate that battery and run both engines and not burn out an alternator - well - for our GB42 1990 - we disabled the battery at the ER switch, put the battery selector into ALL and off we went. The alternator on the side of the disabled battery showed charging, nothing got hot and we made it back to home port.
So - another possible use case. (note as we talked to a few electricians, GBs of different eras are wired differently, and many have been re-wired - so look carefully).