It's in case they go over 14.5V - verboten for Lithium.
The Rolls Royce of Airhead alternators is the 400W permanent magnet one from Silent Hektik (brushless and doesn't depend on the exciter current through the bulb).
Regulated to 14.4V and gives max output at a lower rpm.
They do an ignition and coil system too that does away with the Bean Can.
Great solution, if only someone could find a problem for them!
Never had a problem with the stock system in over 600,000 km on three bikes!
Why waste power generating power you are unlikely to ever use?