I think it went back to early fountain pens. The tips would dry up and you could lick them to get the ink flowing. Habits carried over to pencils and ball points.
I find mechanical pencils to be quite handy.
Those fountain pens were the bane of us lefties. They needed to be pulled to leave a good even ink line. We had to twist our arms around to pull the pen instead of pushing it left to right. That also helped keep our hands out of the wet ink. The ball points were a Godsend for lefties.
