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.
It could be, for example, the risk of a change in the economic environment - what are your contingency plans for that in terms of dealing with such a situation? There may be many other risks as well specific to your particular sphere of operation, but that ability to plan ahead for all scenarios makes for a robust business plan.
Whether it is travel, study, cooking or any other activity involving a process, planning is usually the first step. The same holds true for business. Business plans are probably more important than the business itself. For example, the plan for a house is more important than the house itself, though it is the house that people remember and not the plan. But the house wouldn't stand without the plan, would it?.
Planning is the first and the most crucial step for starting a business. Business Planning lays down step-by-step procedure by which would function. The various functional requirements of business-marketing, finance, human resource and production, provide information to all the concerned people like employees, venture capitalists, etc. and it also draws a read map to help the entrepreneur run the business successfully.