It is bonding. Team building. If the point is won, it is saying good shot - well done. If the point is lost, it is saying never mind - I forgive you - let's win the next. It is annoying though.
It is more like a player messing up a pass and his teammates running over to bump fists with him every time. Doesn't happen. It's popular in tennis because the players are sheep. One team did it and the next and next until you're at the point where they do it because if they don't people will think they aren't a team like the rest. I bet if they were being perfectly honest half the players hate it but darent not do it
In my secondary modern school Digby would have been just Digger ,Dagnall would have been Dagger, Redfearn would have been Red. Phillip Tuffnell would have been Tuffy etc. Now we get Diggers,Daggers,Redders, Tuffers etc it's flippin middle-class cringeworthy imo
They are trying to punch each other but , due to them being supreme atheletes with exceptional reflexes, they always manage to block with their own counter punch .