Nix is a package manager. NixOS makes use of the nice features of that package manager to form a systemd based OS configuration around that.

Nix has a steep learning curve, but nix ends up being useful for all sorts of things related to software packages.

Regarding NixOS, the safety of being able to do rollbacks for any system change is neat. A NixOS configuration is declarative, which means the OS' configuration can be described in a single file. (Or you can compose the OS configuration into multiple modules). -- Declaring OS configuration programmatically in a file is useful if you want to manage more than one or two machines.

That NixOS can be configured from a file also lends Nix to being a tool for building VM images. (e.g. AWS AMIs).

For software development, Nix allows for declaring a development environment such that the host will get the same versions of tools (and libraries) that are declared. -- This is immensely useful for e.g. sideprojects. This is like a more powerful version of asdf-vm.

I myself use Fedora Silverblue with a https://ublue.it/ -based custom image, and I use home-manager for shell configuration. Sure, my GNOME layout isn't declarative, but basically everything else is. Pair that with one of the best NVIDIA driver experiences and the strong feeling of stability, it's better than most other Linuxes rn.

Nix (contrasted with NixOS) can be used on macOS as an alternative to and escape hatch from the Homebrew ecosystem.

You can safely mutate your system instead of using an immutable one.

I'm growing increasingly convinced Fedora Silverblue is the future and most end-user "noob friendly" distros will be using its model by default in 10 years.