Excellent under-body aerodynamics will make the bottom of a car look as flat as the underside of a Hot Wheels racer. But looking awesome is obviously not the only purpose.
Wikipedia does a pretty good job explaining the basic idea. You want to
create an area of low pressure underneath the car, so that the higher pressure above the car will apply a downward force. Naturally, to maximize the force one wants the maximum area at the minimal pressure. Racing car designers have achieved low pressure in two ways: first, by using a fan to pull air out of the cavity; second, to design the underside of the car so that incoming air is accelerated through a narrow slot between the car and the ground, lowering pressure by Bernoulli's principle.
So, although the jacked-up car above may look like flatter than a Ken doll, it's for creating some serious low pressure.