The purpose of the Marketing Plan is to identify and create competitive advantage. Marketing planning is the real form of the company to provide a strategic response to a changing global competition pattern consisting of: business size adjustments, changes in product and market scope, creation of new networking relationships with other organizations.
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.