If you roll back the volume to clean up the tone, aren't you now dealing with a tone that is too quiet to be heard? What if you want a clean tone at the same decibel level as the overdriven tone? this approach wouldn't work, right? Unless you increased the volume on the amp?
If you set the amp up right - as well as the guitar, rolling the volume down is hitting the amp with a bit less signal which is why it cleans up and, if you increase the volume on the guitar, you are sending more signal which cause the amp to overdrive.
Think of the volume as a 'control' for the amount of signal that you are sending to the amp. With the 'volume' rolled down to zero, there is 'no' signal being sent and, if you set your guitar and 'amp' with the guitar volume at max, the only option is to 'roll' down, to reduce the amount of 'signal' that will hit the amp and, if you want to 'boost' that to push the amp to overdriven sounds, the ONLY option is with a 'Boost' pedal. However, if you were to set the amp up just on the verge of break-up, where you can get it to crunch with some digging in, but playing softly, its 'clean' but with the guitar volume on 6/7, you can turn the volume up which pushes the amp into that 'crunchy/overdriven' sound (like you can get by digging in - which sends more signal to the amp like turning up the volume does) or roll down the volume meaning that even if you dig in, you get cleaner tones.
If you learn to use your guitars tone and volume control, you can do without boost and treble boost pedals - you can have a more 'dynamic' response too. At the end of the day, its your Amp that really dictates the 'volume' of the sound you here and the Guitar is just controlling the amount of signal that is sent along the signal path - the more signal, the more it can overdrive the amp so you set the amp right on the edge of break-up so if it receives more signal - either by really digging in or turning the 'volume' up on the guitar - it overdrives and if it receives 'less' signal, either by playing softly or rolling back the volume, it cleans up. You can still use it as a 'mute' too but you now have more control over your set-up.
If you use the tine in a similar way, EQ the amp with the tone rolled down to 7 - may need to add a bit more treble at the amp, you can 'add' tone much like a treble boost does as well as roll it off if you want to as well. By setting up with everything on the guitar set to 'Max', the ONLY option is to go 'down', reduce the signal, can't boost it, reduce the tone, can't boost it. People buy pedals to do what their guitar can do if they set it up right.