In theory Spoilers are intended to do exactly what the name implies: spoil or disrupt the airflow around a car. On a an Indy Car, front and rear spoilers called wings create down force thus decreasing the lift produced at high speeds and improving maneuverability.
Rear spoilers, a close cousin to the wings used on the Indy car, on buyer cars serve a alike function, improving handling. With roots in performance, most spoilers today serve to improve a cars aerodynamics and serve as a stylish accessory.
So, the answer, at least for a manufacturer installed spoiler, is yes they serve a function. What about aftermarket rear spoilers? Do they accomplish the same level of downforce? The answer there’s a certain maybe. If a spoiler isn’t installed properly, and the angle of the blade is off, then it may really be rising drag on the car without rising downforce. If a rear spoiler is too heavy for the car, then the weight increase could offshoot any fuel efficiency that may have been produced by the airflow disruption.
As for how do they look on a new car? No comment 🙂