The mini-ITX is just a small format mother board, a lot of people use them in a htpc. I decided to use mine as a server to supply the NUC end point after reading the thread on AS.
To answer your question, -why not directly from the switch to the NUC - perfectly valid question and it can be done easily.
Because you asked I tried it out today and it was pretty good but not as good as the server feeding the NUC. I have been playing some classical piano and it is more dynamic (with 2PCs) the piano sounds fuller, more solid, woody if you like, better harmonics, transparency and placement. This Chopin CD I ripped a while ago is unrecognisable in terms of quality compared to when I used to play it in my old SCAD machine. As a single PC the playback is more like the original machine with an external HD drive - a bit better I must admit.
I am not particularly computer literate and have not got the hang of all the software and networking ramifications yet.
A lot of the posters on AS use roon on a library server and send the files directly to the NUC renderer (end point). I am not familiar with roon but as I understand it the server does all the heavy lifting.
I don't have roon but Jriver lets me do something similar, however any upsampling is done by the NUC. The general consensus is this configuration is the reason for the uplift in quality, one of the many along with linear PSUs, reclocking just about anything, special switches etc, the permutations are endless.
Just my limited exploration has proved very worthwhile.
Some comments:
You can do a LOT better than Jriver. Have your tried Amarra or Audirvana?
Adding a tx-USBUltra in the USB cable to your DAC will make a big difference, powered from a fast LPS like the SBooster.
I don't know what your Switch is, but the Switch from SOtM is excellent sounding, with internal clock.
Steve N.