Depends on whom you ask. Larry Ruth states in War Baby II: War Baby Comes Home that the snap had no purpose, at least until the early '50's when a few pistol belts were made with a corresponding snap.
Craig Reisch, in U.S. M1 Carbines: Wartime Production, states that the snap lined up with a corresponding snap on the M1910 pistol belt, and that the use on the stock was an discovery made by troops issued Carbines.
Bruce Canfield, in Bruce Canfield's Complete Guide to the M1 Garand and M1 Carbine, concurs with Reisch.
Whatever you call it, stick a piece of masking, riggers' or 100-mph tape on the snap if you put the pouch on your stock. If you don't, the snap will mark the stock's finish.