Aerodynamics plays a big part in car design. This is because a car that has less air resistance will use less fuel than a car with more air resistance (same engine w/ different car bodies). Furthermore, a car with a more aerodynamic body will be able to get to a higher speed than a car with a less aerodynamic body. The Top Gear video shows a car with a body that is not very aerodynamic. On the other hand, you can see that the wind passing over the Mercedes SLS AMG has a much smaller resistive force on the car. Thus allowing the car to move faster and use less fuel with this body rather than the other one.