It helps prevent the heat sink from damaging those bits (metal to components) while allowing the chips' 'face' to touch or come within close proximity (with thermal grease) of the heat sink without damage. This pvc tape-like layer offers protection from the surface components on the GPU and CPU (note the bumps and small soldered on bits on both the GPU and CPU). Granted, this plethora comes from multiple models, but this should clear things up. These are the bits of 'tape' to which you refer I gather.

Paul - just trying to offer a bit of assistance with visuals.