Yeah, that argument is weird. If we are taking about state the difference between those two is that docker is just storing a snapshot of a state (BTW Dockerfile by itself is not reproducible and is comparable to a bash script) while Nix is storing information how to get to a given state. That makes it far more versatile. It is what Docker promised to be, but ended up being an overglorified zip file. Depending on your needs, that might be all you actually want. If you want to make small adjustments to existing state (let say applying a patch, in docker you pretty much have to rebuild from scratch and with non-deterministic Dockerfile you might change more than that), or maybe compose something new from existing components, Nix gives you far greater control to do that.