The first point to keep in mind about business plans is... have a business plan! This may seem obvious but is overlooked. Many people start businesses without a plan; sometimes it can come from sheer bravado, thinking "I don't need a plan", or alternatively you might hear "It's all inside my head, that's my business plan".
When I have received business plans, the very best responses come from people who have looked at the risks and have an answer for every question. What you never want is to throw a scenario at your plan and have to answer "I don't know what I would do in that situation". You want to plan for every possible contingency, and certainly all the major risks to the ongoing success of your business.
The reality is no matter how much you work with things in your head, no matter how confident you may be and how much you think you already have a great vision for your business, there are so many great reasons why you should get it down on paper.
For any organization, various functional plans-marketing, financial, human resources, production-have to be laid down, even though all the functional plans serve different purposes and all theses plans provide guidance and structure to the successful running of the business.