You'd put the things that aren't up to personal preference in there, like an AI disclosure / contribution policy or what changes should look like to be reviewable. You usually don't need to tell humans "don't change things for the sake of changing them" because we don't like to do more work than we have to, but agents do.
You can also include a compact "table of contents," describing what lives where in the project. It saves at most a little bit of exploration though so I don't know if it's really worth the maintenance
You do definitely need some agent-specific guidance locally. The committed AGENTS.md should be pretty sparse. Ironically I think the clearest use case is for instructing agents that AI contributions are not allowed. Not that they won't ignore it if prompted.