FUNCTION OF SPOILER IN CARS EXPLAINED !!

The device we see at the rear of a car is known as spoiler.It is an automotive aerodynamic device whose intended design function is to 'spoil' unfavorable air movement across a body of a vehicle in motion, usually described as turbulence or drag. 

When car is running at high speed, two basic aerodynamic forces act on the car,first one is lift and second one is drag.The lift force tends to lift the car from the ground as a result the grip is reduced and vehicle becomes unstable.

A spoiler is fundamentally an inverted aircraft wing. The wings of a aircraft generates lift which helps the aircraft take off. While the spoilers also generate lift, because they are inverted, they push the car down on the ground.Thus,it increases the car’s grip on the road.

Without spoilers, the only way to increase the grip would be to increase the weight, or to change the compound the tire was made out of. The only problem with increasing the weight is that it doesn't help in turns, where you really want to grip. All that extra weight has inertia, which you have to overcome to turn, so increasing the weight doesn't help at all.Also,increased weight will result in increased fuel consumption.

Spoilers  on commercial vehicles are generally added for styling purposes and have either little or no aerodynamic benefit.