Well, number one, is keeps the user from being able to push the bellow into the reed. If the reed would creep up out of the bellow for any reason (it needs to be seated as close to the bellow as possible, because this allows for the most forceful air movement) the reed would immediately be ruined the first time it was hit.
I prefer it to suck air across the board (like the video is showing you) in the small bellow calls, and push air across the board on the large bellow calls. (I set my reed in the barrel about 3/8" deep because the large bellow moves a lot of air).
Vince