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.
The "Accidental Entrepreneur" Plan: Believe it or not, it happens quite often. An impulse, a hobby, or a passing notion turns into a business without warning. One day you're handing your extra back_yard tomatoes or homemade cake to the neighbors, and before you know it you're filling out the forms for a booth at the local farmer's market. Perhaps you create a unique bit of hand_crafted jewelry and wear it to school or work, and then find your phone flooded with messages like, "Where can I get one?" and "I'll pay you to make one for me."