Your DAC may be powered but its USB input / processor may need to be powered by the input device (i.e. USB streamer) 5v line. Having two separated / shielded wires, one for the data, one for the 5v power line, may benefit your DAC as it eliminates cross talk and other types of interference. Whether you can hear the difference by doing so or not, it's up to individual user / listener, system, etc.
Need help understanding USB cables
Looking to connect a streamer to a DAC via USB (A to B). I am reading that some cables separate the power from the signal portions of the cable but the DAC is powered, so what should I be looking for in a cable?