This is actually a Physics question, but who cares, A spoiler, most commonly called a wing is to add pressure to the back of the car to keep the back tires on the road, thus causing the back tires that propel it to have more traction and grip better, Spoilers are used on Rear wheel drive cars, if you put it on a front wheel drive car it is just drag being added. Spoilers are used to enhance grip so that the driver can get to point A to B easier, also a lot of faster cars like Stock car racing and Drag racing, without it the car would go flying due to all the air under it, it would act like an airplane wing...
Short answer: Air pressure forces the back tires down to the ground more causing more friction so the car can grip better.