Why do you need a business plan? Simply because it is the most important written document that defines the goals to be achieved within a certain period of time through the combined efforts of the company's personnel and the cooperation of third parties.
Yes, it is primarily for you, but not only for you. Business is a complex effort involving both your staff and third parties (government agencies, banking experts, suppliers, investors, etc.) who are ready to cooperate and win with you. They (banks, insurance companies) can provide you with the resources you need (bank loan, investment capital, international trade finance...), but words alone are not enough for them. They will definitely ask if you really have a detailed business plan. As a rule, they need a DOCUMENT to consider your offer of cooperation and make the right decision.
To learn more about Business Planning as a process and Business Plan as a document click here.