Agree with the rest of answers here.

1. The purpose of any business plan is to serve as exercise for you to conceptualize, research and better understand your proposed venture. Once is done, it should serve as quick reference in the future if you are to come stranded or confused with possible paths in operations. There is no need to determine why you need it necessarily, because it has one purpose. If you are going to seek funding, investors want to see this because it shows how much you know.
2. A business plan is no good without a marketing plan. In some cases a marketing plan is all you need.
3. Don't include proformas unless you are required by someone like an investor. Most BP are almost outdated as soon as you finish them because most don't put real effort into a generalist operational business in industry plan, therefore is not a good idea to spend a lot of time in a plan with financials, made up projections, etc if there is no need.
4. Be realistic, even with investors if you trick yourself you're entering a world of huge challenging disappointments. So be real about opportunities, threats, strengths, resources...