The process of creating a system manually, as described in Chapter 5, Building a Root Filesystem, is called the roll your own (RYO) process. It has the advantage that you are in complete control of the software and you can tailor it to do anything you like. If you want it to do something truly odd but innovative, or if you want to reduce the memory footprint to the smallest possible, RYO is the way to go. But, in the vast majority of situations, building manually is a waste of time and produces inferior, unmaintainable systems.
They are usually built incrementally over a period of months, often undocumented and seldom recreated from scratch because nobody had a clue where each part came from.