it's just a volume pot in a box. if your amp has a master volume,
it's the same thing!
if your amp has an effects loop but no master volume, then this gadget would indeed
become the master volume.
more useful is the electro-harmonix
signal-pad, which does the exact same thing but has an on-off stomp switch; that way the guitar player can be loud for his leads, and
turned the hell down for rhythm playing.