Captured thinking: You don't allow your financial people to keep their numbers in their heads. Financial reports are the lifeblood of the numbers side of any business, no matter what size. It should be no different with marketing. Your written document lays out your game plan. If people leave, if new people arrive, if memories falter, if events bring pressure to alter the givens, the information in the written marketing plan stays intact to remind you of what you'd agreed on.
What's the relationship between your marketing plan and your business plan or vision statement? Your business plan spells out what your business is about--what you do and don't do, and what your ultimate goals are. It encompasses more than marketing; it can include discussions of locations, staffing, financing, strategic alliances and so on. It includes "the vision thing," the resounding words that spell out the glorious purpose of your company in stirring language. Your business plan is the U.S. Constitution of your business: If you want to do something that's outside the business plan, you need to either change your mind or change the plan. Your company's business plan provides the environment in which your marketing plan must flourish. The two documents must be consistent.