In my case I wanted to invalidate the notes when rearranging commits, because changing order could introduce bugs, but also have a single command that ensures that all my commits are tested before sending pull request. I think maybe it used to work that way (remove notes upon rebasing) out of the box that time, but if not, then I suspect I simply added the git commit id to the note, which can be effectively used for validity checking.

Of course, if the notes mechanism didn't exist, then I could have just used a local file.. But it's nice to see the messages in the git log.

But yeah, both kinds of keys would be useful for this purpose, depending on the exact needs.