I would think that if you have a number of batteries in parralel on a bus, and you would like to service one of the batteries, for what ever reason, how do you isolate the cables from the other batteries with out a complete shut down?
Having an isolator for each battery, makes this a simple task. Switch the battery off you need to service at the battery control. Open the isolator, and now you can disconnect the cables with out having live wires hanging about.