In any race, the winner gets all the fun. That is why; racers try to upgrade their car once they got the money. They wouldn't want to lose on the next race. They also get some funds from sponsors, but no one wants to sponsor a loser.
Drive your car as fast as you can till you reach the finish line. You can only unlock the next level if you landed in the top three spots. Grab as many coins that you can while driving as you can use it afterwards to upgrade your car. You will also earn stars, which you can trade for new car.