If you feed the computer's 5VDC supply (from the computer's USB port) into the DAC, it might not be as flat as you'd like. Cheap computers regulate their 5VDC supplies to the requirements of the digital logic that makes up the computer.

If you want to provide cleaner power to a USB device, you provide it with an external power supply. Since DACs (or ADCs for audio input) have analog sections, you have to deliver power that meets those specifications.

You might also choose to provide filtering on the USB signal lines if your USB device's circuitry is susceptible to digital noise. USB signals are digital (approximately square waves) which contain harmonics that aren't needed to successfully decode the signal on the receiving end. Filter those out as well as high-frequency transients caused by other devices in the computer.

CAVEAT: I know nothing about the Schiit device.