While I don't use NixOS or home-manager, I would imagine this provides some extra value: i.e. config is versioned or easy to move between machines.

Curiosity got the better of me so I looked it up at https://nix-community.github.io/home-manager/ and it indeed does purport to provide benefits I guessed at and then some.

Whether that's better than just manually managing things yourself is altogether a different matter.