I can give you a simplified explanation and let others who are more familiar with computer audio chime in, but I know the USB interface carries’s a 5V power signal along with the data. Since the power is traveling down the same cable as the all important data, noise can be introduced into the data stream (the music).
There are devices used to clean up the noise, one is known as a "decrapifier," and the end result is cleaner data to be used when imported to the audio components.
There is also the issue of "jitter" which is the timing of the digital stream travelling from point A to point B. That’s why a DAC needs an internal or external clock device to correct the timing of the data stream. Basically, computers and analogue music systems are not a perfect fit.