About the external power supply needed...
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.