The saccular lung, found in snakes, appears to have no role in breathing. It seems likely that it's a 'vestigial' organ from a time whenhad two working lungs. Modern snakes have only one 'working' lung - which is suggested to be an evolutionary trait as a result of their lack of body space.
Some aquatic snakes use the saccular lung to adjust buoyancy whilst swimming, but terrestrial snakes seem to have no obvious use for it.
One current theory is that it might have a function as a 'cantilever' to help maintain body shape. See: Herpetologica Vol. 59, No. 1 (Mar., 2003), pp. 52-57