In short, Nix reduces the setup time, both for your system and for your projects. If you find yourself spending a while setting stuff up (for example, after a reinstall; or maybe you want to run your project on another PC and need to install the right dependencies), Nix will help. Otherwise, if your desktop is vanilla Fedora or whatever and you don’t do much programming (or you don’t have any dependency management problems), Nix probably isn’t for you.