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.
A business plan is essential for all businesses regardless of size, and once one has been written it is also important to maintain and update said plan. That being said, it is important to realize that whilst writing a detailed plan is useful and a good foundation for any successful business, it will not necessarily make the business a success or guard you from all disasters. If you keep up to date with business plan it will prove a really useful tool throughout the lifespan of your business. However, if you grossly over exaggerate profit margins or your budget then your business plan could also lead to failure. Stick to the figures and be realistic and your plan should hold you in good stead in the turbulent business market.