The plastic acts as a water barrier and a wind stop. It keeps the rain water that gets past the window seals from getting to the door panels, then redirects that water back inside the door so it can drain out the bottom of the door and not run inside to the panel and inside the car. It also helps keep outside air from entering the passenger compartment through the doors.

Our pampered cars can probably survive well without redirecting the water back into the doors to drain out the bottom, they likely won't see many hard rains. If you can live with not having everything as factory equipped, the do it your self process will work just great. Gene