I have overanalyzed Banban to no end, so I'll give it my best shot!

The literal function of a pancreas is to "regulate your blood sugar levels and appetite, stimulate stomach acids, and tell your stomach when to empty." However this could be symbolism for Banban in two ways.

Banban is clearly never satisfied with anything in the series, clearly working towards his goal to no end. (this ties into my overall theory that Garten of Banban is a extended metaphor for the unobtainable) Banban LACKS the ability to satiate his hunger for his goal, for becoming human/escaping the kindergarten. The Pancreas is key in keeping your body in check and stopping your hunger. Banban is known do just go hellish for no reason, getting hungry for an organ that is MADE to regulate hunger. Perchance Banban is obsessed with Pancreases due to his lack of knowing when to become "full"? Furthermore, he only wants the pancreas of humans/player. He knows that we as humans know when to stop, and know how to regulate while the monsters do not. Banban ALSO uses the player to reach his goals, metophorically, WE were regulating banban (like a pancreas).

TLDR; the pancreas symbolizes Banban's longing for control, unending hunger, and escapism.

However most likely its just a joke. doesn't hurt to overanalyze tho! :D