I partially agree with you: not knowing what particular functions etc. do is one use case, another for me would be to detangle complicated control flow.
Even if I can reason about each line individually, sometimes code is just complicated and maintains invariants that are never specified anywhere. Some of that can come down to "what were the constraints at the time and what did we agree on", but sometimes it's just complicated even if it doesn't have to be. The latter is something I would love for a LLM to simplify, but I just don't trust it to do that consistently correct.
