Spoilers on sports and performance cars are designed to offer improved aerodynamics by disrupting or spoiling the airflow over the back of the car.
http://en.wikipedia.org/wiki/Spoiler_(automoti ve)
http://en.wikipedia.org/wiki/Audi_tt
Early Audi TT models gained press coverage for a series of high-speed accidents in Europe. Reported crashes and related fatalities occurred at speeds in excess of 110 mph (180 km/h) during abrupt lane changes or sharp turns. Both the coupe and roadster models were recalled in late 1999/early 2000 to improve predictability of the car's handling at very high-speeds. Audi's Electronic Stability Programme, and rear spoiler were added, along with suspension modifications. All changes were subsequently incorporated into future versions of the car.