Half of these exist because I was bored once.
The Windows 10 and MacOS ones are GPU passthrough enabled and what I occasionally use if I have to use a Windows or Mac application. Windows 7 is also GPU enabled, but is more a nostalgia thing than anything.
I think my PopOS VM was originally installed for fun, but I used it along with my Arch Linux, Debian 12 and Testing (I run Testing on host, but I wanted a fresh environment and was too lazy to spin up a Docker or chroot), Ubuntu 23.10 and Fedora to test various software builds and bugs, as I don't like touching normal Ubuntu unless I must.
The Windows Server 2022 one is one I recently spun up to mess with Windows Docker Containers (I have to port an app to Windows, and was looking at that for CI). That all become moot when I found out Github's CI doesn't support Windows Docker containers despite supporting Windows runners (The organization I'm doing it for uses Github, so I have to use it).
I run a different LXC on Proxmox for every service, so it's a bunch. Probably a better way to do it since most of those just run a docker container inside them.
Why mix docker and VMs? Isn't docker sort of like a VM, an application-level VM maybe? (I obviously do not understand Docker well)
Serious answer, I'm not sure why someone would run a VM to run just a container inside the VM, aside from the VM providing volumes (directories) to the VM. That said, VMs are perfectly capable of running containers, and can run multiple containers without issue. For work, our Gitlab instance has runners that are VMs that just run containers.
Fun answer, have you heard of Docker in Docker?