What is the difference between the models writing code "well" and their performance in this context? Are we referring to readability?

Genuine question. If we use agents to read, edit, and review code, why do we care about readability? That's a human constraint. Unless attempting to do those three is not effective and thus requires human attention to correct issues which would justify readable code. If that's the case; why use the agent to edit the code in the first place?