Documentation is important. Even though good programs, or generally, well-designed systems, should be rather self-explanatory, it cannot hurt to attach some information to them. Perhaps to make certain parts clearer, perhaps to explain something more about the concepts and ideas behind certain details.
Having not enough or not the right additional information can hurt the effiency of the whole target program, process or concept. Users do not understand how to perform certain tasks, where to apply certain actions and why things are laid out as they are. Nevertheless, often a comprehensive documentation of what is available and accessible is missing.
One could ask: "Why?"