Knowing what you want to say, seeing it clearly in your head, and then just letting it rip at the keyboard may work with an essay (or a blog post), but with a book the length of the project will mean that sooner or later the words you just wrote will inevitably begin to influence your next line of thought, and soon you’ll be veering off on tangents on tangents. Writing an outline is something every author should confront sooner or later. Sooner’s better, as thinking it through will help you clarify your aims in your own mind. It’s also better because if changes are suggested, they are easier to implement before the passage in question has been written in full.

But it seems so cold and final. Much nicer to let your inner Heathcliff drive you along wherever he wants. Still, beware; if you want to get a publisher on-board, you’ll need to write a proposal indicating why the book’s needed and why you’re the one to fill the void. An outline will be a necessary part of that process: so you’re going to have to do it anyway — may as well get it done as early as possible when it’ll be of most help to you. So all writers, even self-publishers (perhaps especially self publishers who won’t have to go through the disciplinary step of satisfying an agent or editor), will end up benefitting from having to make a thorough outline.

Help is provided by a wide range of sources. This article from Publishing Talk by literary agent Sarah Such focusses on the writing of the outline. A more business-oriented tack is taken by Jane Friedman.