It is a cosmetic enhancement.
If you read some of the early 2007 Aero 8 threads, there were various options bandied about when the sidepipes were introduced; including, but not limited to, a mesh screen that matched the S1/S2 lower grills.
Aeroman, Boshly and I tried to get the factory to make a nice, thin fin, diffuser (at times the suggestion was carbon fibre); however, in the end (during the time of the S3) the plastic diffuser was introduced.
Whilst it finishes the car, I doubt it adds anything to the aerodynamics of the vehicle.
