Who says that stunts can only be done on a car?
Stunts can be done whatever vehicles you are riding. Just remember that you need to avoid injury to do a perfect stunt. As such you are competing in a race and you want to impress your sponsors, so stunts are necessary. Everyone knows that it is not easy to do stunts. Of course, you would get paid from winning the race, but you are not going to get paid for doing stunts. Anyway, it is a great way to impress some sponsor and they might want to sponsor your next race. They would find you entertaining and some people would like to see that. This is a great way to advertise their product even if you didn't win. That means that you don't have to win to make money in a race.
In this game, you need the money to upgrade your chosen character. In this game, each race would be different, so you can choose different characters. The idea here is to win the race, but aside from that, there are also mini missions in each race. The game gets more interesting and more difficult as you make some progress.