Maybe your drive(s) fail and you want to reinstall. Then you already have a setup with all your software and config files installed. Just reinstall NixOS and re-apply your configuration (or build your own Install ISO ).
And if you ever get a new laptop/desktop/VM/VPS you can do the same.

Don’t forget to take backups, regardless of your setup tho.

The reproducibility also leads to some surprise features, like being able to wipe your entire system on every boot. Since NixOS always puts the necessary files in the correct place, this is perfectly fine. If you then add some mechanism to persist specific data across reboots (a separate partition, or the Impermanence module), you will remove all kinda of randomly accumulated files on every boot.

This means I have very small backups, because I have three kinds of data: stuff that is wiped on every boot, stuff that is persisted but not backed up (/nix/store, all kinds of caches) and stuff that is persisted and backed up (documents, repositories, media).

None of my OS’s files are in the backups, which makes of them a lot smaller than my previous arch install.