The main purpose of Covetous Shen, the Jeweler, is to help the player level up the jewels that they find while adventuring. He can also remove jewels from a socket, which comes in handy.
Shen starts out at level 1. Players can raise each crafter up to Level 12.
When I play in Adventure mode during a Season, I try and level up the Jeweler as fast as I can. The downside of doing this so fast is that I miss out on the wonderful artwork that Blizzard created for each level of the Jeweler. There are so many details to see!
As far as I can tell, the Jeweler’s area looks the same for Level 10, Level 11, and Level 12.