There is one main advantage to reducing air resistance on a car. The advantage is that this reduces the amount of drag on the car, thereby reducing the amount of fuel that is needed to drive the car at any particular speed and for any particular distance.
When a car is driven, it converts the chemical energy in its fuel into kinetic energy (motion). However, not all of the chemical energy in the fuel converts to kinetic energy. Instead, some is lost to heat and some to sound. The heat and the sound are caused by friction. One source of friction is air resistance.
If we reduce the amount of air resistance on the car, less of the chemical energy will be converted into heat and sound. This is beneficial because, when we drive a car, we want as much of the chemical energy as possible to be converted into kinetic energy. We do not want to waste our fuel making heat and noise. By reducing the amount of air resistance, we improve the efficiency of our fuel usage. We increase the distance that we can drive on any given amount of fuel. This is very useful for car owners (and for society as well since it means that we waste less of our resources).