Two reasons Pete. First, the rotor draws about 2.5 amps. Considerin' the rather small size of our batteries, that's a current drain that ain't helpful. That current is used to create a pretty powerful magnetic field. The slightly over half a horse starter has to fight that magnetic field as it spins the rotor. Get rid of the field and that's more power freed up for starting. At least that's the theory. Soon as I get a point and shoot tach, I'll try and quantify this mod to see if it's really worth it.
Second reason... bike fires but don't start. That's enough to trip the light relay and turn on the lights. Unless you cycle the iggy switch, there's another couple amps being drained when we need 'em most. At the least, this mod will fix that problem.