The question makes perfect sense to me, and that's good reason to be concerned.

One suggestion is that when people were naked and running about they needed skin on their privates for protection from twigs and insects and sunlight. However once people got into the habit of wearing clothes skin wasn't needed for protection and bacteria became more of a problem so hacking skin off penises could provide some protection against oportunistic and sexually transmitted infections.

While this explanation my provide some rational for standed male circumcism, it doesn't really explain penis splitting or female circumcision. Maybe they got started just to be nasty. I can just imagine someone saying, "You behave yourself or I'll split your penis!" Then after getting his penis split as a punishment the guy started acting tough and boasting about it so it became cool to have your penis split and no one ever really thought about how stupid it is.

Of course the best explanation I think is chance. People have often done stupid thing for no good reason and this is one of them.