My guess would be the deterministic configuration management. If it's for a personal machine then do what you want to make it your own but if it's an organization (e.g. government) you want the machines to be as similar as possible to reduce maintenance. Think cattle, not pets.

from a sys admins perspective it's painfully easy to deploy across hundreds of machines. NixOS is declaritive and immutable. you can take one system configuration of NixOS and throw the same config on as many machines as you want. packages, dotfiles, whatever are all replicated the exact same way on each machine. You can lock all the packages/apps, configs, kernels, etc to a specific version therefore a sysadmin may only need to upgrade the lot once a year. if a user some how breaks their system it's as easy a fix as rolling back to the previous generation with a single menu option.

So basically say you have your computer and you want to replicate your exact setup to hundreds of other computers. with NixOS it's as simple as setting up a git repo for your nixos config, pushing to it, installing nixos on every other computer then cloning your repo to all the machines and rebuilding. done. now you have hundreds of other computers that all behave and work the same way your computer does.

If something needs updating or a fix needs to be rolled out all a sysadmin has to do is fix it on one machine, push the change, pull it for every other machine and rebuild. done.

From the article:

Its use of the Nix package manager means that each package is installed in its own directory with immutable and signed contents. That alone means that the setup is incredibly easy to reproduce across multiple machines, something that is an obvious benefit when dealing with different facets of a government.

Id argue an immutable distro would likely serve the same purpose but maybe its also because nixos isnt as "locked down" so itd be easier to configure for a specific purpose?

But not curious enough to read the article.