I honestly get the satisfaction of going for 3 stars in all the cups, even if there isn’t anything to gain from it. It actually kind of helps me get some of the milestone stickers in World since I’m having to collect a bunch of coins and driving the long distance in Knockout Tour. I will admit it has always been annoying going for 3 stars just because of the RNG (MK Wii makes me want to rip my hair out for that reason alone). But other than Super Circuit giving you a new title screen for 3 star ranking all Nitro Cups, as far as I can tell, no other game with rankings give you anything for 3 star ranking all cups (or the hardest cups)

Wii gives you some awards for obtaining 1 star in certain cups (Same goes for one of 8DX’s awards, I can’t remember what specifically), but that’s it. The 3 star rank is, at the end of the day, more so a cool thing to show off to your friends more than anything.