Helps the maintainer focus their efforts during review. If you wrote that exact sentence, a maintainer would keep "understanding of the codebase" as a place for potential sources of incorrect assumptions higher in mind than otherwise.
I'm not really a developer but I routinely need to go dig through codebases looking for answers and AI assistants have made this so much faster. This has been a boon for me.
In my experience, if there's one thing the AI assistants do really well, it's understanding existing code and summarizing it or pinpointing specific things I'm looking for.