The electrical interference can't do anything to to the digital audio, but interference on the USB cable can potentially be picked up by the analog amplifier circuitry in the DAC. I had a particular combination of headphone, DAC and amplifier years ago that I could hear electrical noise on when no music was playing.
Absolutely. I work as a sound engineer and have used lots of things of this type, and it's particularly easy to understand how a USB connection to the computer would wreck the sound of a cheap (sub-$1000, not professional) DAC.
Optical digital means perfect isolation from the ground plane of the computer. It's that simple. The DAC can do whatever it needs to manage its own noise levels, but you're pretty much guaranteed a huge difference from entirely decoupling the DAC from the computer's ground plane. That electrical interference can do a surprisingly enormous amount of damage to the analog circuitry of the cheap DAC, which itself is probably not very resilient at rejecting any sort of electrical interference.
