Dunst has one by default. I have used it twice. Once when I was very tired and couldn’t output something into a file for some reason (I think it was a full disk) and piped it into a notification instead and once when a program put some error message in there and the notification expired before I read it.

It can also be fun for seeing how your perception of notifications compares to the ones that actually show up. It is however important to remember that it exists and to not let important data end up there.