Side skirts are used to reduce the amount of high pressure area on the side of the car to go under the car from the sides.The side skirt splits the bottom of the car into two parts: One, the underbody which is a region of low pressure and higher air velocity. Two, the side of the car, which is a region of relatively higher pressure. Without the side skirt, the air from the side of the car will rush in to the underbody due to pressure difference causing a reduction in down force. The presence of the side skirt prevents this from happening by behaving as a simple blockage. In sports cars, it is a great way to improve downforce levels. On everyday cars, it is not so effective, it is more like a style statement.