Two benefits to Nix that I can think of:
1. It has a wider range of packages available vs asdf. Asdf can support runtimes that have an asdf plugin, whereas Nix has over 80,000 packages available in it's registry
2. Nix also handles the dependencies for the packages you want to install. If you runtime requires a compiler, specific libraries, etc, Nix will ensure those are installed.
Of course, Nix can be challenging to get started with. If you want to try Nix but find the language intimidating, you can try something like Devbox first.