I use Arch btw
Documented declarative centralized configuration of all my systems; that value can't be overstated.
Then the fact that I can run any version of any nix package from anywhere at any time with a single terminal command,
the ability to effortlessly and cleanly switch desktop environments with completely no friction or fear,
to be able to roll back by booting into any previous configuration still on disk, or entirely rebuild any older configuration I've tracked in git,
Painless and fearless updates with a rolling release, very little chance of borking and I can never get out of step; doesn't matter if I update once an hour or once a year,
I can share my settings with friends e.g. they can try out my MPV config with a single nix command, or consume my flake outputs and add it to their system with merge/overriding changes,
I can clearly version system dependencies for everything that needs them without shipping containers,
I can define development shell environments which is basically like python venvs with requires profiles but for every language,
I can remotely provision a machine, including disk partitioning and luks and initrd ssh, with a single command,
There's a plethora of modules that make setting up complex services with sensible defaults as simple as "enable = true;",
... I could probably go on for a while but yeah