Whata the killer benefit of nix over, like, a docker file or a package.lock or whatever?

package.lock is JSON only, Nix is for the entire system, similar to a Dockerfile

Nix specifies dependencies declaritively, and more precisely, than Docker (does by default), so the resulting environment is reproducibly the same. It caches really well and doubles as a package manager.

Despite the initial learning curve, I now personally prefer Nix's declarative style to a Dockerfile