This very deluxe, plush looking heavy tolex bag is a nice addition to your amp.
It is stiff enough to not need the cardboard under the reverb springs, and improves on the actual "science" of a reverb bag.
Mounting the reverb in a fabric bag isolates it from the vibrations of the amp cabinet, preventing "wolf tones" or sympathetic vibrations.
Since all the amps with short 9" reverb have them solidly mounted to the cabinet, this reverb bag allows you to finally isolate the reverb springs from the cabinet vibrations.