Well, certainly there is bidirectionality on ethernet between router and server, router needs to know server there. The difference is information from music player (Roon in my case) doesn't follow that path, it goes out second ethernet port directly to FMC (in my case) could be streamer or renderer for someone else. Without the second ethernet port information from music player has to go back through the ethernet cable connecting to router or switch.
Isn't a router without wifi a switch, never heard of a router without wifi.