It's used as a grooming technique to agitate the fish, same as keeping two males in a divided tank, or keeping a BP in the tank for the FH to dominate over.
it's to make them mad and pump up. Not only for fhs but good for all cichlids or fighting fishes to keep them perked and alert. And those things lead to a nice looking strong fish.
If you keep fighter bettas before, it is the same principle.