The main goal of side skirts is to create an area of low pressure between the car and the track, generating increasing pressure as the car increases in speed making the car stick to the road. Lots of different aids have been designed around this area, including side skirts to increase downforce.
These are a way of channeling the cars low pressure to the rear of the car and directing it to the rear diffuser. It also prevents high pressure air from around the car interfering with the low pressure air underneath the car. Linked closely to the performance of the whole aerodynamic package.
Colin Chapman is the founder and designer of Lotus and also the pioneer of Ground Effects in modern motor racing. In 1972 he designed the radical Lotus 72, featuring shovel nose cone in the shape of a wedge, also the radiators were located in the sidepods. Winning both drivers and constructors championships titles that Year. This lead to the Lotus 78, with true side skirt introduction, creating a vacuum effect, sucking the car onto the floor with higher speeds. This lead to another World Driver champion shipped with the driver Mario Andretti.
I found this explaination. I doubt if the side splitter on the mustang has this kind of effect but adds looks.
2011 Red Candy
/build 2/1/11/ 401A
MT with Redline MTL/ Brembos/355's/glass roof
Shaker 1000/Scoops/Clear bra/Steeda UCA and PH bar
Redline hood lifts/Bilsteins with Ford Racing P springs & axle bks
UPR catch can/ES shift bushing/whiteline kdt928/Epco ball
Shaftmasters DS/BMR loop & relocation brackets
Nitto 555 G2's/Steeda jack rails