well, for starters, that you don't have to dick around with 2 knobs when the overall volume need to be increased, keeping the balance between channels the same
(my MTS modules often have 'sweet spots' on their channel volume for instance, my master volume allows me to keep them there)


then, there could be the distorting of any gear in the FX loop when setting channel volume too high, and probably a few more reasons that elude me atm

I have 4 channels, and it lets me keep my volumes balanced without spending forever dicking around with each channel volume.

I generally keep the channel volumes close to noon and adjust them as necessary to be balanced. Then raise or lower my MV depending on where I'm playing.

Hell, I have two MVs. Because Marshall.

Gotcha, didn't think of it that way.

Well, here is why you would have a master when you individual channel volumes...

Some people like all their distortion to come from the pre-amp and some people like it to come from the power amp or a combination. If you crank the master so it's working hard and then use the channel volume set fairly low, you are going to get more power amp distortion. Although it's not perfect that way. You still want to hit the power amp relatively hard to get the interaction with the power transformer as well.

It Is because Of different ways that amps are built, thus different circuits, different ways to get tone. if they all had the same circuit they would all sound very similar. The original amps mostly only had a single volume. then musicians liked the way they sounded when turned all the way up and distorting, but they were to loud. So some builders added a preamp with volume control so you could get the distorted signal at a lower volume level. then the musicians wanted 2 channels, then 3 channels, then 4, etc.

Gotcha and thanks for the info. I never thought of it that way. I considered the gain control to be the output stage for said channel , master volume would be overall volume and each channel volume would just allow you to tailor the given amount of channels the correct levels as you need. The ability to take the entire volume overall up or down makes total sense specially since different times and places require different volume levels.
Also hitting the preamp or power amp section for gain is another consideration I didn't take into account. Clears up many questions and thanks guys.