I have one I built that has no leather. It has an indicator style grip and has been changed so much to tweak it that I would have gone thru a whole hide covering that thing. A grip is not required, but it is indeed found on most, maybe because that is just the way things morphed, or maybe because the builders wanted to cover joints (if made from other than a stave). A leather or rubber grip does feel better to me, but not necessarily THAT much better.
Where it shines is, sometimes, my hand slides a bit on a non covered bow handle.
ChuckC