Here there are two pieces of film laying against a back window. The rear window has a compound curve, a curvature both up and down, and left to right. The result is a flat piece of film will not lay flat on a rounded shape. This causes the film to lift off the glass in several places. In the picture there is water underneath the film so you can clearly see the finger shaped excess film areas. This is where heat shrinking comes in. If we don't shrink the film we will have to use multiple pieces, and we don't want to do that.