As a minimum, your written plan will help you in the following ways:

Development - Particularly in startup situations like this, the plan can function as a "to do" list.  It helps you establish strategic plans, set realistic deadlines, and delegate assignments.  It sets deadlines for you.

Management Planning - The plan forces you to consider the interrelationships of each facet of the business and often becomes the operating bible for the management of your company.  This will be of great importance if you decide to grow your agency beyond a one-person operation.