that piece of plastic is called a "moisture barrier" alright. But I've found that it plays another role:

one of my doors did not have that plastic barrier, as some work had been previously done on the inside, and they never bothered to replace it. Ever since, i've been hearing creaks from that door whenever i'm on a bumpy road.

When i installed a new plastic film using a "contractor-grade" garbage bag, the creaks magically went away. The plastic film prevented the creaking caused by the inside panel rubbing against the metal parts of the door.