I've come across this on a number of aircraft - mostly twins and French aeroplanes, but never really thought about it. I assumed it is to ensure less load on the starter motor so it can turn at full tilt - when the alternator is generating power the load on the engine increases (which is also why many sprint racing cars run with no alternator, to maximise power output).....Could be to protect the circuitry as well I suppose, though I don't know why it would used be in some aircraft and not others....
I fly a Rallye with the 160HP engine, they are beautiful aeroplane to fly and it is aerobatic
