I've read and I've been told that using a thumbless grip, especially in pull-ups and pull-down:
It allows lessening the load on the forearm muscles and better targeting of the bigger back muscles – latissimus dorsi (lats).
It is suggested also in this video of Jeremy Ethier about lat pull-down.
But why it happens? I'd say that using a thumbless grip will release the thumb adductor, which is a hand muscle, not a forearm one.