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.
So if you can put this all together, you can have a complete business operating plan that would show you exactly what your business would need to do to give you the income and profit you would like to have and a rough idea whether your market would support it or not. All you would have left to do would be to figure out how to make it all happen.