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.
Funny that you mention Niri, because I've been getting my ducks in a row to switch from Gnome + PaperWM to Niri, and I was wishing it could be as safe as Nix or some of the other atomic options.