Here is my take on the system based on what I read prior to my purchases. I am not a very technical person nor a recognized audiophile (getting there).
I have a MacBook Pro that sits in the bedroom running Roon Core that is tied to my Tidal Hi Fi account.
Roon recognizes the microRendu on my home network as a music server. The MacBook Pro handles the Roon core and I can access the core via any device I have. In my case a MacBook, IPad, and iPhone to browse and select music.
The music selection is sent over the network (WiFi) from the MacBook Pro. microRendu is connected to my modem/router via the Audioquest Ethernet cable. The microRendu is built/optimized for audio playback and nothing else. It takes the signal from the modem/router and then pushes it via USB to the Bitfrost Multibit and onward.
To answer your question specifically, I believe it optimizes the output for USB audio and the "system" is convenient for multi device control.
Additional cost? Yes. Additional convenience? Yes. Better sound? I think so.
That's my answer and I'm sticking with it

i'm guessing someone else here could explain this better but that's my take.
Cheers,
Steve