This film provides two valuable functions. One, it keeps paint and stain, that might seep behind the glass stop during the finishing process, from being exposed to view when looked at the product from the outside. All that is visible is the black film, which blends in with the black glass spacer of the insulated glass unit.
Secondly, this factory applied film provides for quick masking of the glass for whoever is painting or staining the wood components of the product. If you order your products without an interior finish from the factory, this film is left on the interior side of the unit, making it easy to apply additional masking prior to finishing. Note: if you order your windows and doors with a factory interior finish this extra film will already be trimmed away from the daylight opening.