If I understand it correctly, it's there to up the lower end bass and the upper end treble so that it sounds better when listening at lower volumes. It's called loudness or bass boost or contour, sometimes (from what I've seen).
This vintage HK amp I have has a cool feature by which the effect is lessened as you turn the thing up, until it is completely defeated. It actually does make the output sound consistent all the way through the dial... for example, the bass is still present at low volumes versus almost nonexistent with the contour control off.
I've hated the control on every other amp I've had, though.
