some spoilers help smooth wind flow over the rear of car helping with aero and better MPG. I have a spoiler on my impala LTZ and when towing trailer can tell a difference in drag vs my older impala with no spoiler and same body shape. SO i think they do help a bit especially at interstate speeds.
Spoiler in front helps move air around car. Air under the car causes drag and turbulence which is less aero. less aero = lower mpg.
at speeds above 45 MPH aero is working for good or bad. the faster you go the more it works.
Nascar racers use them for downforce, we don't really need that on regular car.
Joestralia, you are right to be sceptical. Most spoilers mount at an angle (or lack of angle) that provides no downforce. Think of these spoilers as you would a ground-effects package, a non-functional item intended for appearance. Then check the price.