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.
The business plan serves several purposes: Enables the entrepreneur to think through the business in a logical and structured way and to set out the stage in the achievement of the business objectives ; Enables the entrepreneur to plot progress against the plan ; Ensures that the resources needed to carry out the strategy and the times when they are required are both identified ; Preparing the business plan ensures that the entrepreneur has thought through the crucial aspects of the venture ; It is a means for making all employees aware of the business direction ; It is an important document for discussion with prospective investors and lenders of finance ; It links into the derailed, short-term, one-year budget.
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.
But do not be too paranoid, make sure you are showing it to people you trust, whose feedback you welcome and can be genuinely useful in guiding how the plan takes shape. Very often when working as individuals we get very close to certain details and miss out a big thing that has slipped your mind.