Aesthetic decision and functional purpose! It provides a spacer to protect paper art or photographs from direct contact with the glass. This is necessary, particularly in humid climates, to prevent the art from coming in contact with condensation or sticking to the glass and permanently damaging it. For example, if a photograph is framed in direct contact with the glass, in a few years, the silver oxi4e emulsion will adhere to the glass, causing unrecoverable damage to the photo. The spacing provided by the mat will prevent this while it also provides structural support for the artwork.