What is the function of that plastic lip that all P5's have under the front bumper ?

 

What you are refering to is the "scrape plate or skid plate". This is for when coming off a steep curb ,or in some cases you've lowered your P5 like so many of us has, very low and hit a nasty bump, this helps protect the base of your car primarily, by making a nasty noise so you know your riding too low!!! Mine is all scratched up!!!

 

Are you talking about the downforcers, front spoiler ducts, etc?

 

Yea, I was refering to the "skid plate" thing....thanks. I thought It had some other function then just scrape or skid.

 

According to some car makers this also pushes air up from the road into the engine compartment creating a over the engine draft that then goes back down the firewall side of the engine,
otherwise I would agree and say its a noise maker
It will let you know when you scrape your main crossbar and when you do it too much and the motor mounts go out you will know why.