NixOS puts your full system configuration in a portable set of files. You can easily reproduce the same configuration on another machine. I also like that instead of accumulating a growing list of packages that I don't remember why I installed I have package lists specified in files with comments, and split into modules that I can enable or disable.

IMO NixOS works best when you also use Home Manager to apply the same benefits to your user app configurations and such. (OTOH you can use Home Manager to get those benefits without NixOS. But I like that I get consistency between the OS-level and user-level configurations, and that both use the same set of packages.) I use Home Manager to manage my list of installed packages, my dot files, Gnome settings, Firefox about:config settings, and so on.

You might be installing packages imperatively with nix profile install or with nix env -i. If that's the case you're not going to see the full benefits of a declarative system in my opinion. I prefer to install packages by editing my Home Manager configuration and running home-manager switch.

I like that NixOS + Home Manager automates stuff that I used to do by hand. A couple of the things that I do or have done are to,

• test an experimental window manager, Niri
• use Neovide (a GUI frontend for Neovim) with a custom patch to tweak font rendering

Now I have that kind of stuff automated:

• Since there was no packaging for Niri when I started trying it I wrote my own in my NixOS config with a NixOS module to set up a systemd unit to run it. Because Nix packages are effectively build scripts, whenever I update Nix automatically pulls the latest version of Niri and compiles it without me having to think about it anymore.
• I use the Neovide package from nixpkgs with an override to compile with my custom patch. Like with Niri my configuration automatically gets the latest Neovide version and builds it with my patch when I update, and I don't have to think about it anymore. I use this overlay to do that:

modifications = final: prev: {
  neovide = final.neovide.overrideAttrs (oldAttrs: {
    patches = (oldAttrs.patches or [ ]) ++ [ ./neovide-font-customization.patch ];
  });
};

You can see that I compile some things from source. That's fine on my desktop, but takes a while on my travel laptop. But I don't need to compile on my laptop because I can use Nix's binary cache feature. I push my NixOS and Home Manager configurations to Github, and I have Garnix build everything that I push. Garnix stores everything it builds in a binary cache. So when I pull my latest configuration version on my laptop it downloads binaries from that cache.