It is simple ergonomics.
Standard rifle stock designs, be they pistol grip or straight like on the Win 94, force you to turn your hand at an angle which it was not really designed to go. Some worse than others.
This is not good.
Think of the militarys of the world. AK-47, G3, M16.....etc....
These are all "thumbholes" in a sense. They just dont have the bridge of wood across the bottom.
I have two thumbholes, and frankly I wish they were all like that.