A business plan is very helpful in showing you start-up costs, what you will need for operating costs and tasks you will need to accomplish in order to reach your income goals. In addition, a plan will help you establish a budget, where you are less likely to overspend and run out of money. One of the biggest reasons that businesses fail is due to lack of funds - you do not want this to happen to your business.
The focus of a business plan is to convince investors and partners that your business idea will be profitable. There needs to be enough data and research that a lender would feel comfortable putting his or her money on the table. Stick to the point and keep it as brief as possible without leaving out important details..