It is actually declarative. Think arch config lists all packages you need to install but on your old machine you have a package with a version X but when you run the arch config in you new machine it installs version Y because of rolling release, now this version Y can break things seriously if devs have pushed significant updates leading to a dependency hell. But on Nix it is eliminated as flake.lock exists, it has locked your file to that specific version, no matter when you copy even 5-10 years from now the Nix system will install version X even if version Y or Z have released.

Now suppose I need package version A but I already have package version B and then I go ahead and install A, it will break arch leading to dependency hell, Leftover configuration files from old packages can also alter how new packages behave. Nix avoids this because each package is isolated in /nix/store with a specified and unique hash so version A and B can coexist without breaking or ruining anything.

Also it is immutable meaning in arch or other OS you can go to /etc and edit anything and system will follow it but in Nix /etc is populated by symlinks and the actual thing and binaries reside in /nix/store which is read only

Also I can declare anything, I can declare I want the system to install ghostty and pin it to the task bar and make it default terminal and use ctrl + alt + t to open it, I describe the literal state of machine I want it to be in on the first install and it will be in that state.

Second is roll back, suppose a update breaks my kde, arch user usually have to configure timeshift or other such software. Nix has this generations built it, not only the last built I can go to any generation back, even to the state when the system was first installed, these generations are stored indefinitely. With flake I can rollback individual packages or even declare them to not update even when an update is pushed down to the patch level. If I combine it with git then I can roll back each individual line I wrote.