Library cabinetry is generally defined as enclosed base cabinetry and open shelving above. These cabinets are customarily built with a back panel which provides the dual purpose or strengthening the cabinet as well as providing a clean looking interior.
There are occasions when one might like the beauty and color of the wall behind to show through. This approach is best suited when you desire a nice contrast for the objects to be displayed. If the shelves were to be fully stocked with books you would want the strength of a back panel provides.