[sparkie]

What Guix and Nix offer over puppet et al is a greater reliability in reproducing an environment - by eliminating the side effects that could lead to errors, or possible configurations and package combinations which have not been tested.

The fun part is that configurations can be treated exactly like packages and vice-versa. A "package" can depend on a specific configuration too. After you realize that package management and configuration management are not really disjoint ideas, it's the only sane way to think about reproducing software reliably.