It reduces the power of the signal. It isn't limited to EQ's, you can attenuate any time through out recording, mixing, monitoring. Like line pads you slap on so signal isn't coming in so hot to a piece of equipment. Basically If a signal is coming into a piece of gear and it's too hot or causing unwanted distortion and the input on your receiving piece of gear is on it's lowest setting, some type of signal attenuator would be needed whether a di box or a simple homemade barrel attenuator.