It’s primarily for viewing orientation, not a special preservation requirement.

On film, the emulsion is the image-bearing side—the side that faced the lens during exposure. When you place negatives emulsion-side down in a storage page, you view them from the base side, and the image appears in the same left-right orientation you expect from the camera scene.

If you insert the film emulsion-side up, the image will appear horizontally reversed when you look at the sheet.

So the instruction is mainly a practical convention: it makes contact sheets and quick visual inspection easier and keeps all strips oriented consistently in storage.