It is also about the way packages are specified: "Nix is a purely functional package manager. This means that it treats packages like values in purely function

It is also about the way packages are specified: "Nix is a purely functional package manager. This means that it treats packages like values in purely functional programming languages such as Haskell — they are built by functions that don’t have side-effects, and they never change after they have been built" (from the About Nix section on top of the page)