A marketing plan is not only necessary for new businesses, as it can be used to help existing programs incorporate the strengths their company currently enjoys in an effort to implement necessary changes or improvements.
A marketing plan can be implemented for a new product or service in which case it is meant to pull together all of the needed elements for an effective marketing start. It is important to have a marketing plan so that you can determine where you are, where you are headed, and how you will get there. It is especially important to have a marketing plan so that you can submit it for things such as loan considerations.
As a whole, you should see your marketing plan as a process for which you have a team whose goal is keeping it simple, developing a time-frame, providing feedback, implementing necessary revisions, and remaining consistent with the mission statement.