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.
Running a business certainly can not be separated from the purpose of doing business is to achieve profit. In an effort to achieve profit, have you had a good marketing plan? Or it turns out your business is running in place without any clear management.