http://sitemaker.umich.edu/saiprasad/files/tecplot_animation_car.gif
The body of your car is important in a number of ways. It's the primary buffer between the occupants and the environment. The body is a barrier against things trying to get into the interior of the car like the outside temperature, rain, snow, dust, flying insects, telephone poles and even other vehicles impacting your car. However, since I believe in this, I will focus more on the performance aspect of the body of a car.
Performance-wise, the body of a car is designed to cut through the air. Just as a parachute falls slowly in air, a large, flat shape moving against air is slow. A smooth, sleek, wedge-shaped object moves much more easily through still air. The shape of a car determines its coefficient of drag. The coefficient of drag is a number which describes how well an object cuts through the air. With your typical truck, it's about .6. A Ferrari or Corvette will be around .3 or less. Most cars are somewhere in between. The force caused by air resistance is a product of its body shape, and it is described by the following formula:
Far = .5
* Cd * Air Density * Drag Area *
Speed2
The coefficient of drag is Cd.
Air density is a measure of how "thick" the air is and
is in units of kg/m3. The drag area
is the total area of the car if you were to look at it
straight on. It is the width of the car
multiplied by the height, so the units are in m3.
The speed of the car is more intuitive, and is
measured in m/s. As you can see, the force
caused by air resistance increases when you have
denser air, a bigger drag area and higher speed.
As air density isn't something a car maker can design
for, companies build high performance cars which
minimize their coefficient of drag and drag area,
usually by creating a low-slung, rounded-wedge-shaped
car. This creates little drag area for the air
to work against while generating down-force, something
which increases the normal force we looked at in the
wheel section of this webpage. This is good
because it increases the frictional force the tires
generate! That said, this shape can result in
some very unfortunate incidents when the right end of
the car is pointed in the wrong direction as seen here.