It looks like it's a tool to help manage hardware passthrough, specifically to pass through a single resource (GPU most often) and have the UI know that all the guests sharing it are mutually exclusive and you can only have one running at a time.
So some use cases would be for gaming (where you really need dedicated graphics performance), and also for MacOS as a guest OS (as otherwise the user interface is really slow)?
Is this something specific to proxmox or non-mac hosted virtualization? I ask because I regularly run MacOS without GPU passthrough without any UI slowdowns at all. In fact, at one point I ran macOS in a VM as the primary UI on my MacBook for several years..
If you don't know, screen sharing is done very well - it does very intelligent things, such as drag and drop files from desktop to desktop or rich-text copy/paste between systems. It will also pass keys correctly, so command-q will quit a remote app instead of quitting the screen sharing app itself. same with alt-tab.
I used to use screen sharing with the macbook itself, but it kept going to sleep with the lid closed at inopportune times even with caffeinate, so I just cut to the chase.
I also was able to migrate the macos vm when I upgraded my proxmox machine hardware. "vzdump <vmid>", copy the dumpfile, "qmrestore <dumpfile> <vmid>", voila
This was on a late 2013 though, so the experience may be very different now, in fact now that I've replaced it with a 2020 iMac, I'll give it another go and see.
I have a lot of difficult to explain this. I will improve the docs... maybe record a video.