Exactly as others have said, it's just a visual reference for where the modelled power tubes start adding power tube distortion, like a cranked master on a real amp would also do. Because the Axe uses a custom taper for the master knob and a somewhat compensated output level from it, exactly where the "volume" stops going up and the "distortion" starts increasing can be tricky to find. Whether that helps or hurts your tone is for you to decide. I think mostly it's there so if you pull up an amp and it sound bad, you can really easily see if it's got all sorts of power amp distortion on note. More specifically, Mesa Rectifier amps fall apart when they don't have power amp headroom, so it's a really quick visual aid to see where about your should be on the Master knob.
Nothing is "technically wrong" in music. And those are amps that are famous for needing to be loud enough to peel off your skin to sound their best. Lots of useful tones throughout the Master Volume range on them!