A utility without documentation is of no utility at all.
A system component's documentation describes the utility of that component, whether it's a device driver, an executable or, most importantly, a game. Although there are plenty of documents available on how to read Unix documents, or where to find them, few focus on how to write them.
This document serves as a reference guide to writing Unix documentation. If you add something to your operating system, whether it's a new file format or directory structure or device driver, it needs documentation.