Its primarily a souvenior -- most players are not using them and are putting them in their pockets (there is some fear that it may accidently get pushed out into play when the player doesn't want to go all in).

I have previously read of the idea of using a button to mark an all in bet where a player doesn't want to push a large stack of chips but those ideas generally have the all-in buttons in the dealer rack so the players don't walk off with them.

Today i had one player repeatedly use the button and it didn't feel natural to me. It was silly because this player had a small stack and he was putting the stack out anyway after throwing in the button.