Our 240V mains supply was never designed for sensitive electronic equipment but rather lighting, heating, and motors. As a result the emphasis of the power supply companies is on reliability, stability, capacity but not quality. This means the mains power supply is not pure, with interference not filtered out, voltage fluctuations common, and control signalling injected into it. So hi fi manufacturers pay particular attention to the design of the power circuits in their equipment to try and eliminate the uncertain power they are plugged in to.
The supply of power should be considered another important component in a high quality audio system. This category includes a variety of devices to lessen the effect of poor quality power.