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.
Top-level reflection: In the daily hurly-burly of competitive business, it's hard to turn your attention to the big picture, especially those parts that aren't directly related to the daily operations. You need to take time periodically to really think about your business--whether it's providing you and your employees with what you want, whether there aren't some innovative wrinkles you can add, whether you're getting all you can out of your products, your sales staff and your markets. Writing your marketing plan is the best time to do this high-level thinking. Some companies send their top marketing people away to a retreat. Others go to the home of a principal. Some do marketing plan development at a local motel, away from phones and fax machines, so they can devote themselves solely to thinking hard and drawing the most accurate sketches they can of the immediate future of the business.