It becomes easy to communicate your ideas to the people you interface with, who in turn can give you better ideas or complement the ones you have. Collect all the material to one place. Having a business plan makes it possible to have all your ideas and plans in one place. It becomes easier for you refer to it and to use it to develop your business. You become a more committed business person and even others people, like your prospective financiers, can easily regard you as a serious business person.
Developing Planning Modules: Compartmentalizing your plan by developing planning modules or "chunks" allows you to attack the plan in parts, yet still maintain a cohesive plan. I have found that developing an annual plan made up of quarterly targets _ thus becoming a rolling quarterly forecast financial model _ allows for a cohesive structure along with the nimbleness to react to market conditions. At the end of each quarter, a true_up process to align results to annual targets needs to be re_forecast and adjustments made.