It's made for sports cars traveling at high speeds. Here is an explanation from a BMW website: "What is the purpose of a rear diffuser? The primary purpose of a rear diffuser is to efficiently increase the downforce of a vehicle. This helps increase grip and reduce aerodynamic drag. IMPORTANT PART TO UNDERSTAND? In order to fully understand the principles of how a diffuser works there is a simple rule of physics that must be understood. Air pressure naturally moves from high to low. Think of inflating a tire, high pressure exists in the air tank (100psi) and low pressure exists in the tire (30psi). Once the valve is open on the tire the pressure moves from high to low, the tank to the tire. This simple content must be understood when learning how a diffuser works. How does a rear diffuser work? As a vehicle drives down the road, high-speed air is passing underneath the vehicle at low pressure. As the air passes to the rear of the vehicle through the diffuser, the air expands through an expansion chamber (from high pressure to low). Through this expansion, airspeed reduces and pressure is increased (through what is called the Venturi effect). This pressure differential between the low-pressure air under the car and the high pressure at the rear creates a vacuum sucking the air out from under the car. With high-pressure air above the car and low pressure under the car, high downforce is present while reducing aerodynamic drag."