If you have something easy to deploy like a go binary you can just write a dockerfile but for big python projects that start to compile dependencies that is quickly no longer true. The dockerfile likely is also not matching the software you run and test on your local machine, so sometimes debugging is not as easy. Ofcourse you can debug inside the container but then you are missing all your tooling and need to bring that with you. And rebuilding a dockerfile is often not reproducible, so if you want the container back from 1 year ago and you no longer have the artifact you are probably out of luck.

With nix you can easily open a shell with the packages used in the docker image or go back in time and reproduce that image from a year ago with the flake.lock from a year ago.

Also applying patches to dependencies used in dockerfiles is not dead easy as with nix.