There are a few reason I have seen them for:
The first as the first poster state is a car-club. This is common among antique car owners.
The second is awards for concourses, though people who actually win at these would never put them on their cars, as it would detract from the pristine conditions they must be in to compete.
The third, which I see most often is mileage marks. Mercedes used to issue one if you hit certain mileages in their vehicles. My neighbor hit a half a million miles and got a medallion for his vehicle.
There may be others, but those are what come to mind off hand.