If a human wrote it, I can be fairly sure that the human meant it to do what it is advertised as doing. If I run it through its paces and it seems to work, that gives me some confidence that the person who wrote it understood the problem and solved it competently, which allows me to guess that all the parts of it I haven't tested are also likely to work.

If an LLM wrote it, I know that it is a stream of plausible bullshit generated by a statistical model in response to my prompt, whose function may or may not have anything to do with my intent, because the machine which emitted it lacks any capacity to have intentions or to reason through the consequences of its choices. If I run the code through its paces and the basics seem to work, that tells me absolutely nothing about the rest of it. All I can count on is that the code will appear to be plausible, and I have no way of knowing without careful review whether the assumptions baked into it are sane and fit for purpose.