(1) I made a guess based upon my general knowledge about injection molding that the void is there to control the wall thickness of the molded part. If you've talked to Glock engineers who say it's there for a different reason then they're more likely to be right.

(2) As a datapoint, my Kahr CW9 and Springfield XD45 both have similar, albeit narrower, voids between the rear wall of the magazine well and the rear wall of the grip.