It's like a system-wide home-manager alternative for Darwin systems. Or the other way - a NixOS-like whole system management that works on top of Darwin.

For some examples: you can manage your homebrew packages, set keyboard preferences, trackpad settings, updates, finder defaults, etc through the nix config.

Check out its manual to discover all of the configuration options you can set.

https://daiderd.com/nix-darwin/manual/index.html#sec-options