In this case its parily a souvenior. However the case for an all-in button focuses on thiose times when a player makes a verbal declaration of all-in and a player at the other side of the table doesn't hear it, or hears it as "call" unfortunately there are many players who think whispering and mumbling is an effective means of communicating. So Player in seat 3 makes a small bet, player in seat in9 whispers all-in and player in seat 2 trhinks he said "call" and then announces a call. Now the guy in seat 2 is very unhappy when he finds out he just called a large all-in bet rather than the small bet he thought he was calling.

The idea was that you could require a player to actually push out all his chips, but if a player has a lot of chips this can be waste of time. So the idea of an "all-in button" has floated around as a possible solution.