It (obviously) leverages Nix, which in turn means the environment is declarative and fully reproducible (not "reproducible" as in docker). Now, you can use just Nix's devShells, but with devenv you have a middleground between just Nix package manager and a full fledged NixOS module system. Basically, write out one line of code - and you've got your Postgres, another one - full linter set up for whatever language you're using, etc.