When you connect a PC directly to the DAC you are opening up a pathway for PC noise to get into the DAC. Not good.
Your Lumin isolates that network noise and sends a clean signal to the DAC. A PC added to network can also add noise to the network. Think also of all the other things connected to a network, printers, computers, security cams, etc.. All of this can add noise to a network.
I think the new Lumin U1-X has a fibre optical connection. My old Lumin X1 had a fibre connection. A fibre cable is made out of glass and glass cannot carry the noise that I described above. Fibre is better than RJ45 Ethernet connection for the same reason.
I use a cheap very internally noisy computer as my ROON CORE server. The way I have it setup with fibre optic cables makes my system sound great in 3 rooms of the house. My $500 ROON CORE server is actually residing under a bed in my guest room, nowhere near my audio systems.