I was looking at one of my Print File sheets and started to wonder about the 'Insert Emulsion Side Down' statement (not visible in linked page). Are there preservation reasons for this or is it just a convention for the convenience of the users? I believe the sheets are 100% polyethylene.
Simple: so that when looking at the front of the print file sheet, the image appears correctly.
The emulsion side of the film was the side facing the lens, the side receiving the image to expose. By inserting the emulsion side down, you are looking at the back of the image, just as you saw it through the lens, in the same orientation it was in the camera.
If you inserted the films emulsion side up, the image would be inverted horizontally.