In testing and racing cars are constantly changing their speed and direction.
They therefore do not stay at a constant height and angle relative to the track.
External factors such as wind speed and direction, temperature and track conditions are also always changing. It is therefore difficult to make accurate measurements under these conditions.
In a wind tunnel the speed of the air can be kept constant as can the position of the car.
Accurate measurements can be made here and the effects of making small changes to the car can be seen and recorded.