Originally Posted by SysGhost (Unregisterd)
I'm not sure if it's the main purpose, or just a coincidence:
The power supply needed results in a sound quality even better since that power often are clean from any signals/noise picked up otherwise if it had taken the power from the motherboard.
Many times a motherboards 5 volt supply aren't "clean" enough, after it has taken a path through all the other hardware on the motherboard. This kind of noise can also be heard on laptop computers and sounds something like data traffic through a telephone-modem.
That kinda sounds like an issue with poor supply isolation in the analog portion of the sound card. Basically the digital hash from thousands of signals on the motherboard is leaking into the analog outputs of the card. A problem that could have been solved at factory level with a simple resistor and capacitor to pre-filter the supply for the analog sections.