Your business plan should present what a banker or venture capitalist expects to see, in the order they expect to see it in. Following a standard business plan outline will keep you on track, and save you from botching your best chance at getting your business funded.
Preparing a business plan will help you work out the goals you want to achieve, and the strategies to achieve them. This means you can focus your resources and energy on what you need to do, rather than spreading yourself too thin. The planning process also helps you to consider possible bumps in the road and put a plan in place to better manage them if they do come up.
A business plan is a formal statement of business goals, reasons they are attainable, and plans for reaching them. It may also contain background information about the organization or team attempting to reach those goals.
Size your business plan to fit your business. Remember that your business plan should be only as big as what you need to run your business.