The hinge is to allow the cover of the scanner to be lifted off of the scanning bed. This allows a document to be placed upon it.. The hinge also makes it easier to replace the cover without shifting the document once it is placed on the bed. Occasionally, the cover is hinged in two places, one at the back, to attach it to the scanner housing and a second hinge a few inches along the cover. This allows the cover to be folded, allowing a thick object to be placed upon the scanning bed.
