In the CLA pack it's just to add more realism, 50 cycle hum to sound like a unit in Europe and 60 cycle hum to sound like it's in the states. The noise is based on interference that occurs from the power supply and the voltage running through it. It doesn't effect the actual signal, it just adds more realism (I had a convo with a Waves rep about this). I'm not specifically sure about the SSL plugs, but my educated guess is it's the same thing.