The plastic film with the sticky pad is to help with keeping the backplate in place. Not that it helps that much though.
I had to use 1 hand to hold on to the backplate behind the motherboard, 2nd hand to place the water block on top of the cpu and a third hand to screw the 4 screws to the backplate. Only after screwing the water block to the backplate will it hold tight. I'm using LGA1151 socket.