tapping reseats the round to the back of the mag, making it less likely to slip and jam when you send the bolt home. doesnt do too much to help, but old habits die hard. also shakes any dirt in the mag loose, making it less likely to work its way into the chamber.
dont try to tap a full mag on your head if youre not wearing your kevlar. ive seen it done, and its always worth a laugh.
and unless youre really slamming that thing, youre not gonna have to worry about damaging the mag. although they are cheap pieces of junk.
active duty marine armorer