Comment by ndriscoll

Nix does more than software install/updates. It also manages system wide config (e.g. files in /etc, systemd units, timers/cron jobs, firewall, etc.) or even user preferences with home-manager. You can also organize your config into modules to have e.g. a base for everyone + special settings for different BUs that need it. And it does all this declaratively with the ability to rollback to previous working config if needed since it can handle multiple versions of packages all being present until you choose to garbage collect old generations of the system config (which you can also set to automate, of course).

It's basically what people think they want from Docker but don't actually get from Docker.