Reproducibility. Once everything is set up, you can copy your exact configuration, with all the packages and tweaks, to another system via a few configuration files. If your system crashes, as long as you have your configs, you can do a clean install and apply your changes in a few commands.

The other benefit is that it's immutable-ish. You can make non-permanent changes to test out new software or customized configurations, but none of that is permanent until you make it so; even then, you can roll back to a previous version if needed.

But I agree that it's a huge amount of effort for those (arguably very good) benefits. I've tried several times to wrap my head around it, and I just can't seem to get over that learning curve, partly because I want to get using my system right away.

I only have 3 installs of NixOS already, two for work (a server and laptop), and a third for a personal use (desktop). I largely started out by copying a public config from the community. Now I just copy a few kilobytes between the three from time to time via git repo to keep all my kernels, kernel modules, patches, net config, package versions, and system users/groups permissions in sync via lock files. And my user dot files too, but you don't need nix for that last one.

If I have to do something multiple times, like copying a distro across even a few systems more than once, then I figured I'd just revision control it via lockfiles like any other software project.

That all said, NixOS is a hell of a rabbit hole; great for lazy admins and hobby tinkering alike:

https://xkcd.com/1319/

The top graph reflects my stable work install for robot software development,
but the bottom graph is my personal install for hobbies and home lab.

I've distro hopped from Debian, to NixOS, to Arch Linux. The neat thing about NixOS is that 99% of the system and user configuration was spread out across a handful of manageable files. And it was only multiple files because I modularized them; it could've been a single file. Localizing the configuration made it easy to wrap my brain around it.

I've lost track of what I've configured on Arch. I could've been more diligent and kept track but NixOS is more conducive to that from the getgo.

Another neat thing is that nixpkgs (the NixOS package repository) has everything, close enough for me anyway. In one place. I'm already relying on the AUR (a separate repo from the core Arch ones) and all that entails. NixOS (nixos-unstable) is also more bleeding edge than Arch if you're into that sort of thing (I am).

The entirety of your configuration being in one (esoteric, but simple) language was also neat

It wasn't without its downsides but I had fun with it. I totally get the hype.

much more effort than copying a normal distro

I just throw an LLM at it - I learned to read nix (the language), but even that isn't required.

I now maintain a single, mostly shared, configuration between three machines. If I ever replace my main PC again, I can be up and running almost exactly back to where I was within 24 hours (that includes OS installation, some debugging, etc). And there's going to be a bunch of downloading data from my NAS.

That would've used to take a week to get roughly back to where I was, but without some vague fix for some issue or another.

Summary: It's great for programmers, and people who have maintain multiple machines, and want a shared configuration.

Either you pay the setup effort upfront (Nix) or you do it afterwards (any other stateful OS).