A business plan has three primary functions:
Action Plan. A business plan can help to move you to action. You may have been thinking for years about starting a business or engaging in some venture, but the process may seem too daunting, too large and too complicated. A business plan will help you to pull apart the pieces of starting a business and examine each piece by itself. So instead of one large problem, you have a sequence of smaller problems. And by solving the small problems, the large problem is automatically solved. So writing a business plan can help to move you to action by breaking down a seemingly insurmountable task (starting a business) into many smaller, less intimidating tasks.
Road Map. Once you have started your business, a business plan can be an invaluable tool to help keep you on track and moving in the direction you want to go. In the hurley-burley of daily business, it is very easy to lose sight of your objectives and goals -- a business plan can help to keep you focused. A business plan can also serve to help others to understand your vision, including suppliers, customers, employees, friends, and family.
Sales Tool. Perhaps most importantly, a business plan can serve as a sales tool. You will probably need outside financing to start your business, and a business plan is the tool you need to convince investors to come on board. You may also want and need concessions from suppliers or customers -- a business plan can help you get them. Finally you may need to convince family members, or even yourself, that your ideas will bear fruit. A well-written business plan can serve to sell people close to you on the benefits of proceeding with your concept.