Leather is a cool look for my motorcycle jacket, but I'm not so sure about on a bow...
Nah, I'm sure it is a carry-over from the other bows of the string family, and also that plenty of German bows have been (and are) played with an overhand grip. My bow shows plenty of wear from just such use even after the grip disappeared and I even do it myself on occasion.
Overall I'm just more curious about why we're still sticking with the grip today - if there's a legit reason for it or not. For all I know, it isn't possible to get enough weight at the frog end of the stick without making it too thick - or bulking up the frog - and the grip is just a means of adding mass in a way that looks traditional? I suppose a bowmaker who specializes in bass bows might know. Seems a breed few and far between...