They produce downforce to counteract the lift effect of air moving under the car.
It is only necessary at very high speeds, and prevents loss of grip in (especially) the rear tires.
But, yeah, horse and buggy. I'm with you there.
The downforce is there so the car can go around corners quicker; the downforce increases the friction (Ff<=mu x Fn), i.e. the force of friction is less than or equal to the coefficient of friction (mu / μ) multiplied by the normal force. In the case of a tire the normal force is the downward force (provided by the wings or spoilers). Therefore the more downward force the higher the friction hence the more the tire can exert lateral force (cornering force). For example a F1 car at high speed can exert approximately 4Gs of lateral force when cornering, or 5Gs of force when breaking (at slower speeds a F1 car can't brake as hard).