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.
A plan like this would show you how much sales your business would need to do, what your fixed and variable expenses would be, what your material cost, labor cost and profit would need to be to provide the income and profit margin you want. You can see pretty quickly if it's possible for you to get your business to that level. I don't know of any better way than to have your business give you the income and profit you want. What's neat is you can determine what you want your income to be and your profit to be over the next few years and develop a plan that can show you exactly what your business would need to do to give you that income and profit.