There are a lot of things Drupal is good for beyond public use. I could build a system with which I track things I need to do. I could keep an inventory of books and other things I have. Those aren't systems I want to put up for the public, but they'd be handy to build into a website for my own use. Each of those projects, along with many public functions I'd like to build into this and other sites, requires some sort of plan for how they will be used before I can really get started on building them.