Because... well, if I have to be honest, I use the DAC in the UHA-6S Mk.II only because it's convenient.
Toslink is better than USB, and from my brief test, I can tell that coaxial is slightly better than toslink (probably because I have a crappy plastic optical cable), but... I must admit that there are better DACs out there that the amp section of the UHA-6S Mk.II can accommodate.
For instance, I have a Fiio D3 (the first generation one with Wolfson DAC) that I think is leaps and bounds better than the built-in DAC of the UHA-6S Mk.II in imaging, soundstage, and bass articulation. But it's cumbersome to carry around... as I need a separate USB cable for the D3 since it's USB-powered, plus I need the optical cable and an RCA interconnect as well. For all it's worth, UHA-6S Mk.II + an extra optical cable is much less crazy to lug around.
But honestly, I'll echo what some others have said here: if you're looking to get the most out of the UHA-6S Mk.II, it's best to couple the amp section with a better DAC.
But both the D3 and the UHA-6SMk2 have coax inputs, so that a USB to SPDIF adapter could theoretically work in both scenarios. Is it the relative size or cost of the converters and/or the flexibility of the coax vs. Toslink cable in a portable configuration? What am I missing? Simple choice on your part?
Do you have any experience with the Cirrus CS8416 DAC chip in the newer D3?