The difference is the hope of getting out of that situation. If you've inherited a messy and incoherent code base, you recognize that as a problem and work on fixing it. You can build an understanding of the code through first reading and then probably rewriting some of it. This over time improves your ability to reason about that code.
If you're constantly putting yourself back into that situation through relegating the reasoning about code to coding agent, then you won't develop a mental model. You're constantly back at Day 1 of having to "own" someone else's code.