As part of shoemaking, when closing an upper or inseaming, one has to pull two ends of your stitching cord tight. Many of the illustrations show a shoemaker in the process of swinging their arms out to pull their stitches tight. But, as you might imagine, this repeated action of pulling tightly on stitching cords against bare skin can wear down the skin, causing blisters and cuts. The palm, or hand leather, is typically worn on the left hand and is used protect the shoemaker from the constant stress of tugging stitches tight. In this 16th Century German sculpture of St. Crispin and St. Crispianus, altar figures from the shoemaker’s altar in the Marien Church in Delitzsch, a palm is visible on the left hand (what may look like a narrow palm on the right is actually the cuff of the sleeve).
Interestingly enough, evidence for hand leathers in the medieval arena is somewhat scant.