You put in there what packages you want installed, what users, what SSH keys, what services systemd manages.
Then, you run `nixos-rebuild` and then you switch your system to that configuration. The old state of your system stays around until you run garbage collect. So you can rollback to it, if you want.
How often are people doing this, though? I get a machine set up pretty quickly (install list of packages, systemctl enable <foo bar baz>, reboot), and I'm set for several years.
I'm quite new to NixOS and recently tried home manager, which basically manages your home, primarily all dot files, but goes as far as preinstalling favorite extensions on Firefox for example.
I can already do that with Kickstart for rpm distros, though.
Seems like it comes much easier to most people to issue series of (often nondeterministic) commands that mutate state rather than learning a new language that allows describing dependencies and have it figure out steps needed.
The issue to me is that it's mostly a regression and that when showed nix/guix people would squint saying it's horrible.. when in fact docker way is horrible, it's just nicer to them.
Alas, such is society.
If all you do is apt-get install docker, that doesn't feel like a very complete set up to me.