What does a spoiler do?

Answer 1

It keeps the car from coming off of the ground when going really fast.(in race applications with more than 30 degrees of deflection.)

Typically most spoilers on OEM cars are meant for drag co-efficiency. Not traction or road handling. Although some increased performance may occur it is initially designed to deflect enough air to decrease wind resistance (drag). So less duty to the engine to maintain speeds and gas efficiency above 40mph. As speed increases in theory so does the co-efficiency of the airfoil/splitter/spoiler/wing. In layman's its removing or disrupting the air from the surface of the car to move efficiently.

Answer 2

A spoiler on a vehicle helps reduce lift, improve traction, and enhance stability at high speeds by redirecting airflow. It can also improve aerodynamics and fuel efficiency by reducing drag.