When browsing through the oem parts directory looking for mudguards, I noticed that some include side air dams and some don't. What are the side air dams for?

also some say w/ aero or w/o aero, but I'm guessing those are the air dams mentioned above?

http://www.mitsubishiparts.net/cart/...x=0&submit.y=0

click on lancer

 

.... bump can someone help on this

so I can put them on my car sooner

 

bump again.. come on guys i need to know what a side air dam is thats all.