I mentioned the financial aspect of a plan earlier, so let me add this. Another fact about financials to consider: not all business activities are about making money. Point being, in most enterprises financial considerations are centric to the document. But there are some other considerations. For example, a few years ago I wrote a plan for a new subsidiary that was focused on developing an inventory of patents. The potential financial returns were years into the future. Those patents may or may not ever have commercial value. Another example is a non_profit enterprise that has need for a complete roadmap for growing their profile in a market, of which a marketing plan would be the centerpiece.
At this point, you may be tempted to skip writing a business plan altogether, viewing it as an unnecessary exercise in jumping_through_the_hoops, suggested by some old business professor who probably never held down a "real" job anyway. Maybe it's okay as an assignment for an MBA class, but it would be just too confining and irrelevant for today's fast_paced business environment. Anyway, you're ready! You've thought about this business venture for a long time and talked it over with friends and everybody agrees it's a great idea. Best to strike while the iron is hot!