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.
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.
Have you done your research? Do you really know that the people you will be targeting want your product / service, and crucially do they want it AT THE PRICE that you will be offering it at? Whilst confidence in your plan is fantastic, you must make sure that it does not lead you down a blind alley along a path that is not desired by your target market. Don't assume what customers want, do your research and make sure that is clear from the start in your business plan.
Contextualise Your Budget : Of course your budget will be extremely important. But sometimes people sort of pluck figures out of thin air, not giving it the context it needs in the business plan to make real concrete sense of how that budget is going to work.