You can concentrate so much on essential financials and supply logistics, but overlook other issues like marketing or opening times. By showing the plan to someone you trust, they can have a look and see what might be missing or worth developing more. Getting that valuable second opinion on how robust your idea is will put you in a much better position to start and keep going successfully.
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.
A business that makes sense and is going to be sustainable in the future is one that has that clear vision within which all the smaller parts contribute to make it successful.
A valuable source of information about your business that may be required by third parties ; Improved use of your company personal resulting from participation in the business planning process ; Increased employee motivation ; Increased profits and sustained growth.