I think that helps me understand. What are some examples of things where I'd want initialization or registration? What packages are impossible to install with this, besides cases where npm is used as an alternative to apt/yum to install dev executables?

Create registry entries in a config file for all local printers found in the existing OS configuration. Remember that the installer runs with privileges that the application won't normally have. So anytime you have to use those privileges you don't do it at runtime, you do it at install time. And this requires the hook.