It's for switching versions of your local CLI tools quickly.

> asdf is a CLI tool that can manage multiple language runtime versions on a per-project basis. It is like gvm, nvm, rbenv & pyenv (and more) all in one! Simply install your language’s plugin!

For example:

    $ asdf global python 3.6.2
    $ python -V
    Python 3.6.2

    $ asdf local python 3.9.1
    $ python -V
    Python 3.9.1

"asdf local" stores the version to use for the current directory across shells, which is neat. "shell" is for the current session, and "global" is for the system.

The supported CLI tools is extendable via plugins: https://asdf-vm.com/#/plugins-all