The primary purpose of car spoiler is to spoil unfavorable air movement. Its an aerodynamic device, most often fitted to performance cars or sports cars. Car experience two forces lift and drag to reduce these two forces spoiler is installed. Spoiler helps to reduce vehicle lift which results in better traction control as well as better handling.
Air streamlines travel above the car while some air streamlines travels below the car body. Streamline above and beneath car should form a laminar throughout the length of car but in real life this doesn’t happen. In real life front Airflow has laminar but airflow turns into turbulence due to sudden decrease in car shape. There is a sudden decrease in rear end of the car where air pressure drops. Car experience low pressure above the car and higher air pressure below the car and hence car experiences lift force which may lead to loss of traction. Velocity of air travelling above the car is higher than air travelling below the car. A spoiler helps to maintain laminar airflow. Less drag helps in good mileage.
Performance vehicles and sports cars have active spoilers. Active spoilers can be adjusted dynamically when vehicle is in motion and according to condition. Active spoilers can retract, hide and also extend upward when vehicle exceed certain speed. These are additional benefits offered by active spoilers over fixed one. They give cleaner and uncluttered appearance to car.
Passenger vehicles spoilers are fitted to enhance car aesthetic but if spoilers are not installed properly it may lead to increase in drag ultimately poor car mileage. Spoiler fitted for aesthetic purpose could make worst aerodynamic. At low speed spoilers on passenger vehicles actually increase the drag.