>>1084497 (OP)
>Is the purely functional part actually useful or just a meme?
It's useful, one of the big points of using functional languages is to get rid of side effects.
A pure function should give the same output every time for the same input.
This concept is used for NixOS configs, the same config will produce the same working OS environment, this is how NixOS excels at reproducible builds and reproducible systems.
I use NixOS for my servers because once I have that configuration.nix there I can just back it up and know that I can wipe the box, put in the same configuration.nix and be in the exact same setup as I was before.