Golf 6 dust testing video
 

During some of the higher contrast background shots,we can see the entire wake revealed.https://www.youtube.com/watch?v=eD5uAxCXo4A

Freeze at 0:26

What is the purpose of this dust test? Is it used to determine how much sediment is picked up in the door jams?

purpose
 

It's new to me,but must be one of the obscure things an automaker will evaluate.Consumers living on unimproved roads may have expectations about weatherstripping with respect to dust intrusion into the cabin when the windows are up and ventilation is on.
For me,the dust images depict the airflow in a way not possible inside a wind tunnel.