Why do Soldiers and Marines tap magazines on their helmets?

An overview of an often-seen magazine check. Why tapping the back of your magazine is useless and possibly detrimental. Prove this to yourself with the following Range Test: Next live fire session, purposely tap the FRONT of a filled magazine prior to loading instead of the back. Shifting the ammo the "wrong" way like this will have no ill effect, demonstrating this procedure is useless.