These past few days I have discovered something about the volume knob (much credit to Shredcow for imparting this to me as well).
It can be useful tool in sculpting the dynamics of your tone, just as useful as your tone knob. Unless you are someone who rely on a constant tone setting on your amp or pedals, the volume knob can be a cheap boost to drive an amp or pedal, as and when it is needed.
Say for example, in your performance playlist, you need a mild gain tone for one song, a heavily driven tone for another song and both tones for a third. Instead of toggling with the amp knob (and turning your back to the audience), or needing a booster pedal that cost dollars ++-- and risk having to colour your tone, just play around with volume knob. I personally am discovering a new pallette of tones from playing with the volume knob for the past few days.
The disadvantage of it is that on unmarked knobs, rediscovering the volume level's sweet spot can be extremely tedious.