Now that I know it's the matte protector you prefer, I am able to determine more information.
It's PET film, Polyethylene terephthalate aka polyester aka plastic recycling symbol #1.
This means that it has to do with light polarization. A common problem with smartphones and photography for general people is that when they are wearing polarized sunglasses and turn their phones from portrait to landscape it goes black and they can't see the screen anymore. The solution in these cases is to introduce a film to change the polarization, or to get non-polarized sunglasses.
This video clip demonstrates the effect of introducing a polyester film (overhead slide projector sheet) when filming through polarized sunglasses.
Pilots know about this issue because many windscreens and cockpit instruments are polarized, so they need to use non-polarized sunglasses.
around the 2:30 mark of this video you can see them demonstrating the effect on the cockpit instruments.