It's a list of the rules you care about following.
You need to know what to look for, and so does software. Proofreading software or AI is only as good as the list it uses to search for errors.
If you don't use AI, this is less true, because you can store information ambiently. But it's still true—the ambient skill is hard to quantify, and changes based on mood, energy levels etc. We're also distractible, we work collaboratively, and we often need help to focus. Building a reliable and complete list of what you're looking for helps keep the ship steady on course.