Without it, all a package can do is drop files on a filesystem. Its used to do any sort of setup, initialization or registration logic. Its actually impossible to install many packages without something like it. Otherwise, you end up having to follow a bunch of install instructions (which you will mess up sometimes) after each package gets installed.
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?