The killer feature is declarative system management. Reproducible systems is just one of the resulting properties. You want to just try out KDE for a week coming from gnome? Good luck getting rid of all the bloat when switching back on arch. You want to run a program once but not necessarily have it installed on your system? You can do that with nixos. You messed something up and your system now doesn’t boot? You can go back to a previous iteration with nixos, no need to find your liveUSB to start messing with chrooting and stuff. Ever find yourself asking where the configuration file for is so you can edit it? The answer is /etc/configuration.nix Ever had to merge older configs with newer ones because the software updated? (If no, you haven’t been using arch for long) why would you need to do that? You declaratively specified how you want your system to behave and nixos will figure out how to translate that to the new config.
And that’s just the “killer” features I use on a day to day basis