I used to get a notification every year for some dude who had posted on reddit that he was alone on his birthday one year. He stopped replying several years ago, but I still messaged him each year at the prompt of remindme. So that's also a use case.
Damn, I wonder if it's the same guy. The post I originally saw him on had a bunch of people hopping on to the remindme thing, but its impossible to know how many followed through.