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.
Ideally, after writing marketing plans for a few years, you can sit back and review a series of them, year after year, and check the progress of your company. Of course, sometimes this is hard to make time for (;there is that annoying real world to deal with);, but it can provide an unparalleled objective view of what you've been doing with your business life over a number of years.