[jandrese]

You have a Linux box but you want to play a game and it doesn't work properly under Proton, so you spin up a Windows VM to play it instead.

The host still wants access to the GPU to do stuff like compositing windows and H.265 encode/decode.

[skykooler]

And outputting anything to the screen in general. Usually, your monitor(s) are plugged into the ports on the GPU.