It's also nice if you want to run a GUI application in someone else's sandbox.
https://github.com/jstrieb/ctf-collab/blob/9300c57364f71fe29...
I think Microsoft, for a time, offered VMs for such testing... Or maybe it was a third party... Guacamole world be a very good gatekeeper in this situation so the end users don't need virtualization themselves.
If the network is otherwise set up correctly, you can reasonably safely run windows XP behind Guacamole. Of course, I don't recommend that, but it prices that you can write software and then see how any of OS or setup runs your code/website/software... Without worrying that your VM might be compromised in 60 seconds.
As an aide, the siding of virtualization is also helpful for schools... A Chromebook can access guacamole to get to a VM... But a Chromebook can't run a VM itself.