Has to do with Dolby Vision and VRR(for PC but who knows if I'll ever use the C9 again for that now that I have the C2), but for 4K discs and Streaming the receiver I have (Onkyo TX-NR646) only supports up to HDR10/60 Hz and Dolby Vision really makes the C9 come alive. For now, I've got everything plugged into it and using eArc to send the audio over to the receiver. However, some apps will not send Atmos that way even though the TV, and receiver both support it. Ironically enough I can plug either the PC and Disc player into the TV and both of them will push Atmos through eArc with no problem. I still have the player and the Shield plugged into it but the Shield doesn't always send Atmos over, it's an app thing.
Ideally, though, you are correct that the best option is to plug everything into the receiver. That's what I was saying before but for now, I am limited to the age of what I have and an odd set of circumstances that prevented Onkyo from continuing those very, very, successful product lines. They made two more gens that I know of, then switched to a different naming schema, and then got sold. After that, I have seen any real continuations of the product lines I like. I blame soundbars.
I'm in no rush but I'm sure everyone knows what it's like to not have their systems running at full optimization. Thankfully the gaming rig is now.