What is a spoiler on a car spoiling?

The vast majority of spoilers are just for looks. The real function of a car spoiler was to keep the rear end of a fast-moving vehicle, on the pavement. Since, none of us go over the posted speed limit, its just become a fad.

It is spoiling someone’s reputation as a logical person… for the reasons @john65pennington mentioned.

What gets spoiled is the tendency of the air flowing over the car to generate a lifting force that would pull the car away from the pavement.

A spoiler is designed to favourably improve the flow of air over the car reducing drag. A spoiler is not designed to force the rear end down on to the road. The device that you fit to a car to do that is called an aerodynamic wing (or commonly just wing) although the term wing and spoiler have become interchangeable they are very different devices.

We have a winner; @Lightlyseared nailed it.

@john65pennington Unless you go on the highway. Around town, they are just for looks. At speeds as low as 45 mph, they start to make a difference. At 60, you can feel it if you have a light car like mine (2240 pounds). Tennessee does have highways, right?
You are correct that many are just for looks, and those often make handling worse on the highway!

My Z-28 was as stable in excess of 85 mph as it was at 30. They work. The speedometer only went to 85.

From here

“Spoilers” are called such because their function is to “spoil” the airfoil effect created by the shape of a car moving at high speeds. The rear end of many cars tend to lift slightly at high speed, which can cause instability and even poor handling.
Spoilers should not be confused with wings, which, while often referred to interchangeably with spoilers, serve a different function and often look very different. While spoilers help reduce the lift caused by the airfoil effect, wings create downforce at high speeds. That is, if functional, they actually push down on the car at high speed, to help improve grip and handling. Such aerodynamics are extremely common in competitive motorsports and sports cars. Some car makers (such as Gumpert, referring to their Apollo) go so far to say that their aerodynamics create so much downforce that the car could theoretically drive upside-down in a tunnel.”

Speaking of driving upside-down, you haven’t seen spoilers or wings until you’ve seen the Pikes Peak cars from Suzuki, like the SX4.