Sections of a Business Plan
While brainstorming isn't necessarily "part" of the business plan, I think it's a good exercise to run through to get the creative juices flowing. Uninhibited brainstorming allows you to throw out as many ideas as you can think of (regardless of feasibility). Just throw them out there and worry about analysis of the ideas after the brainstorming session is completed. It's especially good to do this with several people, as this will facilitate diverse thoughts... and you may come up with a pivotal idea that will help your business carve it's way into a new niche market.
The remaining sections are, and should be, part of your business plan. Whether you're seeking financing through a bank, a venture capital firm, or any other method that exists out there, they will want to see that you have thoroughly evaluated the business you are pursuing. Statistics are not in your favor (a majority of businesses fail, and it often takes an entrepreneur several failed businesses before they find the "right" one). Showing you have fully analyzed the components that make up your market is critical... not only for investors to see, but in the process of completing your research you may find that the business you hope to pursue really isn't a solid business venture (for any number of reasons). On the other spectrum, in your research you may find that there is a specific market out there for your good or service that you didn't originally see; the process of writing a business plan will help with focus and potentially reshape your idea.