Two reasons: one is to ensure that the rounds are lined up against the rear of the magazine. This is to ensure a smooth feed. The second is, if the mags have been loaded for a while, dirt, etc can get into the spring, causing a mis-feed. Slapping the mag, loosens that dirt up helping the spring provide proper tension.
AFA using your helmet, that's for dramatic effect. I always just tapped mine with the back of my free hand as I loaded them.
BTW, fully loading mags, especially if they're not going to be used anytime soon is a bad idea. It can cause spring fatigue; better to reduce the max capacity a couple of rounds-just to be sure that d&( thing's going to feed when you need it. Source: https://answers.yahoo.com/question/index?qid=20070811185318AAQpDMD
> **Posted by blue9075**
> What's that tap thing you do with the reload animation :c?