So I was going through the datamined BG3 dialogue transcripts and I found this description at the top of the CAMP_Jergal.html file:
"Jergal, a god of the dead, awaits the player in camp. He is bound by the judgment of another god - Helm - to assist the players as penance for past crimes. He provides resurrection services to the players."
I think pretty much everyone already figured out that Withers=Jergal, but that bit about Helm making him assist us as penance for something was news to me!
Edit: I found the backstory in CHA_Crypt_Jergal.html file:
"After the party has slain a group of undead scholars, the judge of death--Jergal--climbs out of his sarcophagus to greet them. He has been locked inside for years by the god Helm to atone for his part in raising the Dead Three, wicked gods who are now plotting to conquer the realms. Jergal is emotionless and excessively formal, both resigned to his fate and that of others. He is here to assist the party because that is his divine oath, not out of any true desire to act. Players can attempt to goad or attack him, but to minimal effect, as he is functionally immortal. His only goal was to see their faces, so he can find them at camp later to offer further assistance."
So it's basically a "get your fucking kids in line" situation.