Digital Analogue isolation /separation

I've read many of the threads on separating or isolating digital and analog AC lines. I still don't think I understand this. If the point is to eliminate digital noise from entering your analog line, does it make a difference when you listen to analog (TT) when there is no digital source playing? I guess if I am listening to digital through my analog amp and preamp I can see this but don't get it the other way??