What is good about NixOS (and GuixOS) is that they apply to package management the same principles that Git applies to managing source code. The Nix store is basically an append-only database (you might even call it a “blockchain”) of inter-dependent packages.
So from an individual computer user’s point of view, it is much safer to install and roll-back software with Nix than with an ordinary package manager that might allow you to accidentally delete package dependencies and break your system. With Nix, you can install packages that actually do break your system, but because of the append-only nature, you can actually roll-back the install automatically right from the Grub boot menu, no need to re-install anything.
Another advantage of NixOS, though this is more from a system operator’s point of view, is that you can guarantee reproducible builds. If the package you have installed has the same hash on all of your computers, that is a simple, human-verifiable proof that all of those systems are running the exact same build of the software. You can probably see that this is very useful for people running servers, like compute clusters, or doing things like A-B testing.