I imagine when starting osu, you can open a chat window that will have notifications if any of your friends beat your score on any map. If you're online and a friend beats a score, you receive a notification like someone sent you a message. My friends have fun competing over weird maps, older maps, easier difficulties etc. Without explicitly being told your score was beat, you'll never find out unless you randomly click the map and hit the friend scoreboard. If a friend actually goes and beats my old scores, it gives me a reason to try hard on a map I normally would never play again, try beating it with mods, etc.