A particular car I worked on just popped in my head. It was a 2008 or so Pontiac Grand Prix. I had put a new door on it and did a water check and looked inside to see water POURING Into the interior on the seat and carpet. I mean POURING in there! I didn't have the door panel or moisture shield on but couldn't believe that the water would be POURING in there as it was. Every car I had ever worked on the "moisture shield" was just that, for "moisture" so the back of the door panel wouldn't be subjected to mildew from the water in the door, but it wasn't designed for actual water to be hitting it, it was a wax paper material with a little glue around the edges, certainly not water tight.
Well all this changed for me with this Pontiac. I pulled the other door panel off to see what the heck it was I did wrong and where was this water coming in. I did a water test on the other side with the door panel off but not the moisture shield. The water was HITTING this moisture shield like a water hose was being sprayed on it! The water then ran down the shield into the bottom of the doors and out the drain holes designed into the doors by the manufacturer.
This "moisture shield" was a thick plastic and it was held to the door with butyl tape, the same material that windows are sealed on many cars. This "moisture shield" was literally a water tight shield to keep water out of the car!