Hi Kelly!
If you go to the Decware ZSTAGE page
https://www.decware.com/newsite/zstage.html you'll see some information. And if you download and open the Owner's Manual there is a very interesting and informative section describing the benefits of the gain stage and the "relaxing" of the sound caused by the use of the tube. (If you're only using a phono source, you may not need this?)
In general I'd say the benefits are: tailoring the dynamics, tonal signature and "weight" of the sound, improving on the sound of your source. I don't have one myself, but have the CSP2 and the ZBox, both of which share characteristics in how they influence the system sound.