My understanding of why the quality of the streaming source or server matters is because of the EMI that these components can generate. General purpose computing devices such as PCs and Mac aren't focused on minimizing the amount of electrical noise that they generate. That has been my experience from going from a laptop to a dedicated streamer as well from hands-on experimentation. A reason to get a separate streamer & DAC is that you might like the sound of one company's DACs but they might not make the best streamers. Some things I've read about digital sound quality from people that should know:
Bob Stuart, Meridian Audio"Of course digital bits-are-bits and with due care, each of the three interfaces (USB, Toslink, coaxial) can deliver the same data at approximately the same time. But the audio we hear is analog and real-world devices are subject to a variety of interferences including data-induced jitter, other process-induced jitter, (and) common- and differential-mode electromagnetic noise. In the ideal world, the data are clocked in by and buffered in the DAC (asynchronous mode) and then de-jittered before conversion. In my experience this can never be perfect, just made closer and closer to irrelevance."
Gordon Rankin (introduced the digital audio world to asynchronous USB transfer)
When I transfer a file over USB to an external hard drive it doesn’t make transfer errors – the file at the destination is the same as the source – so why should sending digital audio over USB be any different? https://darko.audio/2016/05/gordon-rankin-on-why-usb-audio-quality-varies/
A simple model of EMI-induced timing jitter in digital circuits, its statistical distribution and its effect on circuit performance, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 45, NO. 3, AUGUST 2003: https://eprints.whiterose.ac.uk/640/1/robinsonmp2.pdf
Bob Stuart, Meridian Audio"Of course digital bits-are-bits and with due care, each of the three interfaces (USB, Toslink, coaxial) can deliver the same data at approximately the same time. But the audio we hear is analog and real-world devices are subject to a variety of interferences including data-induced jitter, other process-induced jitter, (and) common- and differential-mode electromagnetic noise. In the ideal world, the data are clocked in by and buffered in the DAC (asynchronous mode) and then de-jittered before conversion. In my experience this can never be perfect, just made closer and closer to irrelevance."
Gordon Rankin (introduced the digital audio world to asynchronous USB transfer)
When I transfer a file over USB to an external hard drive it doesn’t make transfer errors – the file at the destination is the same as the source – so why should sending digital audio over USB be any different? https://darko.audio/2016/05/gordon-rankin-on-why-usb-audio-quality-varies/
A simple model of EMI-induced timing jitter in digital circuits, its statistical distribution and its effect on circuit performance, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 45, NO. 3, AUGUST 2003: https://eprints.whiterose.ac.uk/640/1/robinsonmp2.pdf