Correct, it's an adjustable clean boost. It was provided as an additional tool for those players who find the need to sit a little forward on stage during some portions of a tube and don't want to dig in harder for tone reasons. Conversely, it can be used in the opposite way if somebody wants to dig in a little harder without "getting louder", some may find it handy for switching between basses of different sensitivities, etc. Different players will find different applications for this feature and some will find no use at all.