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.
Every business plan should focus a lot on the business's potential competitors, because research and analysis of the competition effectively gives you plenty of useful information. It may guide you as to where you should be advertising and marketing, or certain strategies to use or ones to avoid because you see they have been used unsuccessfully by others.