What is the purpose of a spoiler on the back of a car?

Spoilers are added to cars and other vehicles to make them more aerodynamic. It's function is to 'spoil' unfavorable air movement across a body of a vehicle in motion, usually described as turbulence or drag. Most spoilers are attached to the rear of a car, but they can also be found on the front. Spoilers on the front of a vehicle are often called air dams.

The term "spoiler" is also know as "wing".The  wing is a device whose intended design is to generate down-force as air passes around it, not simply disrupt existing airflow patterns. As such, rather than decreasing drag, automotive wings actually increase drag.Various types and the positioning of the spoilers can do different things to improve a car’s performance. However, the main reason people install spoilers is to allow for better airflow over and around the car, which in turn creates better grip or traction on the road. 

Cars that run at high speeds often encounter control problems. That’s because at high speeds, the increased air flow creates too much lift,which can be especially dangerous when the car makes a turn,as this can make it fly off the road and lose control.
One solution would be to increase the weight of the car to keep in on the ground.

However, this will cause the car to slow down and consume more fuel. Additionally, when it comes to racing cars, manufacturers can’t really afford to add additional weight that can reduce the car’s speed.

Therefore, car makers need a way to counter the lift without adding more weight. A spoiler can reduce this lift without the need for more weight. These devices change the airflow going over the car to increase the downward pressure, essentially pushing the car down to counter the lift.Creating the right mix of lift and downward force can keep the car relatively stable while still maintaining high speeds.