My understanding is that USB audio allows you to pass your music data to an external portable DAC.
But why do I want to do that if my DAC can already store songs? Shouldn't I just store my songs in my portable DAC and play from there and ignore my smartphone?
Disclaimer: I am completely new to portable DAC. I've never tried to set up USB audio out. So please bear with me if what I said is a bit off from the reality.