I think the main difference is that it’s not a tool to turn your config into something that works against a specific target, but instead a collection of examples.
E.g. the example I’ve added so far show specifically how to make a VM that tests the effect of my GRUB PR grub: Add `boot.loader.grub.extraGrubInstallArgs` option by nh2 · Pull Request #85895 · NixOS/nixpkgs · GitHub (or other such changes to the bootloader), and recently I helped building a VM for a friend to make a reproducible environment for building the code coming with his scientific paper publication, which could also be added.
In other words, it tries to solve the problem of “to see how can I use NixOS VMs to solve my problem, I first want to look at some existing examples”.