it's replicable and "atomic", which for a well-designed modern package manager shouldn't be that noticable of a difference, but when it's applied to an operating system a la nixos, you can (at least in theory) copy your centralized exact configuration to another computer and get an OS that behaves exactly the same and has all the same packages. And backup the system state with only a few dozen kilobytes of config files instead of having to backup the entire hard drive (well, assuming the online infrastructure needed to build it in the first place continues to work as expected), and probably rollback a bad change much easier
you can (at least in theory) copy your centralized exact configuration to another computer and get an OS that behaves exactly the same and has all the same packages.
As can you with grepping your history of apt commands? And back up that to a file of a few dozen kilobytes too.
This is completely different from electron. Nix dependencies will be shared if they share the same hash. Electron just blindly copies everything over every time.
Electron just copies over the browser engine required to run it, from a package management standpoint, assuming you're using nom or yarn, it functions the same as nix.