Ground is one _ half of the signal that your HiFi is attempting to reproduce with accuracy and realism. It is the part that the other half, the hot signal, needs to rigidly reference to, in order to create a truly accurate and fully detailed sound stage. Grounds between components should be at zero volts with no voltages present (noise). However cables, metal cases and tone arms all act as antenna for noise, collecting it from the air and by capacitive and inductive coupling with other cables, internal circuits and components within equipment. The most expensive cartridges, DACs, pre amps, and amplifiers cannot deliver a truly faithful performance if the ground is moving, shaking and wobbling with the noise that is perpetually present on every ground line.