On many operating systems you keep accumulating junk around your system which become ever harder to clean up. Many techy people re-install their system every few years to clean it up, many non-techy people simply watch their system become ever slower and filled with cruft until they end up buying a new device.

This is the main issue NixOS solves for me. I don't have systemd units or cronjobs or random config files spread across the system. I have one config file/directory, which contains everything.