plastic behind door panels

I need to install my door panels, they have not been on since i bought the car 2 years ago. I remember when i took the door panels off of my barracuda that there was plastic behind them. I assume this was to keep them from getting soaked with water that came in from the window. I wasn't sure if it needs plastic on the back, or if it was just something that some one did. Does anyone else use plastic?

Thanks,
Steve