Just like NIN said, those are "gain knobs". To be more accurate and precise, those are "input attenuator knobs".
I tend to run mine (on any general amp) in the maxed-out position, which effectively removes them from the signal path......unless there is some problem, then I dial them back until the problem is gone. Typically, "some problem" would be an artifact of poor gain structure and would manifest as either tweeter hiss or high noise floor.
Another reason you may choose to dial them back would be to limit the max possible volume SPL to avoid a chance of speaker damage. I would tend to think people with children would be the most likely to use it in that manner.
EDIT: To really get the best benefit of them, it would be ideal to measure your inputs/outputs along the signal path to properly set the gain structure to avoid clipping. The average user doesn't posses the tools nor the knowledge on how to do this, so it usually ends up as trial and error.