It’s whole deal is “declarative” system configuration. Essentially, it means if your config file is identical to someone else’s, your systems will have identical software/dependencies, and everything should, in theory, run the same, generally speaking.

So for instance, to install a package, instead of running sudo apt install nameofpackage, I would just edit my NixOS config file to have the line pkgs.nameofpackage in it, run a command to “rebuild” my system using sudo nixos-rebuild switch, and it would automatically be installed.

That’s not the whole of it of course, but that’s just a general overview. It’s really good for if you’re running multiple systems that need the same software, because all you need to do is copy the config file over, run sudo nixos-rebuild switch, and the systems now have identical software.

Oh yeah, and you can also easily rollback. If you break anything, you can, while starting to boot NixOS, just select the previous configuration, start your system, and any changes you’d made to software/settings will just be undone. It’s great for troubleshooting.

AFAIK NixOS also has the largest number of supported packages out of any distro.