The Tesla Model S is not just a normal sedan. It is a sedan with supercar performance, 0-60mph in 2.5secs for the P100D. The Model S aerodynamic is designed very carefully and achieved a low drag coefficient. However, with all low drag cars, downforce maybe sacrificed and therefore cornering performance will also be low.
The front splitter provides a double deck design which allows higher downforce but yet not compromising too much drag thanks to wind duct to the ground effect panel.