Do you really need that much downforce?
Because Front-Engine/Front-Wheel Drive vehicles are lighter in the rear, it is important to add more downforce in the rear to achieve more balance. Rear wings and spoilers can do both of these effects.
If a race car is constantly lifting, it does not have the traction and control it needs to go fast and those are some of the key components to faster lap times. Cars need controllability and traction to retain higher speeds through turns. If a car cannot retain higher speeds through turns, then time ultimately is lost.
Two effects are created from aerodynamic wings, downforce and drag. Downforce gives more traction and stability but creates more drag. More drag will slow down a vehicle at high speeds. Adjustable wings can help balance these duality forces. Instead of having a wing at a fixed amount of downforce/drag, adjustable wings allow for more or less of the two. If you are driving on a high speed track, you would want less drag to allow your vehicle to have the maximum amount of top end speed. Conversely, if you are on a technical track with a lot of turns, traction is more of a concern and more downforce would be needed.
Basically, What a rear wing is supposed to do is keep the rear end down in quick high speed turns (at the expense of a getting little more wind drag). Many people crash when making quick turns at high speeds... they spin out. A wing aids in keeping the rear end down at high speeds.