ebay link.....

That's what it does. It lets you crank the amp, and turn down the volume.

Think of it as an extra volume knob... You turn up your amp's master, and turn down the box. The box becomes a second master volume.
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Those "attenuators" like in the link you posted don't help you get powertube saturation at lower volumes. All they do is act like a second master volume, so they are really only useful for amps without a master volume, or if your master volume is too sensitive, so you can fine tune the volume
Most master volume pots are after the phase inverter and are dual-gang so they can compensate somewhat for the tone difference they cause. This thing just looks like a volume pot in a box, which is fine, but it won't even begin to approach the effectiveness of an attenuator or even a half-decent master volume.