For me it’s the fact that I have one source of truth for my whole system config that I can stick in git
If I want to clean up software I don’t need anymore I just remove them from the package list and they’re gone next rebuild
Also means when I reinstall or setup a new system I just run the installer, do a git pull, rebuild and I’ve instantly got all my tools, configured just how I like them
Also, if I want to make a big change I can build my system in a VM first to make sure it works first (not that I do that because it also lets me revert to an earlier build from grub if I need to)
I’ve also got both my laptop and my PC on basically identical configurations from the same git repo with each of them having a smaller config file for hardware specific stuff