Whats are the holes for!!

[Ron Wood]

The follower is the metal platform inside the magazine that pushes the rounds up. The button that you push down on to load the magazine is fastened to the follower. In order to keep the button from working loose from the follower, when the magazine is first assembled the button is "staked", i.e. the end opposite the button is expanded in the follower by a punch inserted through the hole.
I showed a regular magazine with the hole (bottom magazine in the photo) and the rare "flat button" magazine without the hole (top magazine). The flat button magazine was used with the very early Lugers in order to insert them into an unrelieved frame. The flat button was difficult to use in loading so a higher button was implemented and the frame relieved to accommodate it. This higher button is the one that was staked through the hole.