Preparing a business plan will help you to set clear objectives for your business and clarify your thinking. It will also help to set long term goals for future performance and monitor progress. It should help provide an early warning when you might need to reconsider the plan.
A business plan should not only be for internal use, it can help you to secure investment and funding for your business growth.
You should always bear in mind that anyone reading the plan should gain a clear understanding of what your business is today and what it aims to achieve in the future.