A purely functional package manager, distro, devops. And pretty soon, home directory management. Maintaining servers or doing aggressive changes becomes very easy. There's even a Darwin (macOS) implementation now, so you can manage most of your Mac functionally (with heavy usage of defaults under the hood).
It's still lacking a bit on usability, as some things are not as intuitive as in a simple imperative distribution such as Arch or Alpine. Especially if you need to run some prepackaged software that assumes FHS and binds dynamically to some pre-existing dependencies.
But to me, right now if you are a moderately advanced user I don't see a point in running distros that are stuck in the middle (imperative, complex, lots of defaults). It's either one extreme (simple and imperative, e.g. Arch) or the other (functional, NixOS).