What actually happens is that Nixpkgs usually bundles one version of $THING at a time, but as a user you can use, say, 19.09 as your primary system, but get you

What actually happens is that Nixpkgs usually bundles one version of $THING at a time, but as a user you can use, say, 19.09 as your primary system, but get your compiler from unstable because you need some cutting-edge feature, without worrying about dependency conflicts or unintentionally upgrading something else. Or maybe your FPGA programming software depends on some ancient version of QT from 2016, so you pull that Nixpkgs in for that package only.