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.
Rallying point: Your marketing plan gives your troops something to rally behind. You want them to feel confident that the captain of the vessel has the charts in order, knows how to run the ship, and has a port of destination in mind. Companies often undervalue the impact of a "marketing plan" on their own people, who want to feel part of a team engaged in an exciting and complicated joint endeavor. If you want your employees to feel committed to your company, it's important to share with them your vision of where the company is headed in the years to come. People don't always understand financial projections, but they can get excited about a well-written and well-thought-out marketing plan. You should consider releasing your marketing plan--perhaps in an abridged version--companywide. Do it with some fanfare and generate some excitement for the adventures to come. Your workers will appreciate being involved.