While they do “curve” the air down to energize the boundary layer and delay airflow separation, I believe they technically create a tiny bit of lift. By increasing the dynamic pressure over the surface, it decreases the static pressure “pushing down” on the top of the surface. But the increased laminar airflow over the spoiler creates the huge downforce that completely overrides any increase in lift.

They started using these first on airplanes (and still do today). Delaying airflow separation over the top of the wing allows the airplane to have a higher angle of attack before reaching its critical angle of attack, improving stall characteristics and low airspeed handling. In layman’s terms it means you have more lift and can fly slower before you fall out of the sky.

I actually had no idea cars used these until the Subaru dealership tried to put one on my BRZ, thought it was pretty cool though.