The title says it all. You can alter the bias with a switch to move the 'snarl' in the sound to low or high and you can dial in the resonance point to around that point to nail a sweet spot. It is a key feature of more then one pedal.
As far back as the famous 60's Tonebender where there were over time v1 thru to v3 it was deciding on where to leave the bias. It is much better to allow the user to decide.
The boost itself has 35dB of boost. 12 is considered healthy. Much of what it does is a 'feel'
thing and is more audible and effective at performance levels.