Generally, companies are broken down into four levels of hierarchy, the corporate, the division, the business unit and the product level. The overall plan of the company is designed at a corporate level. As part of this planning, each resource is allocated to each division of the company. Next, division level planning allocates resources to each business unit of the company. In addition, business unit planning designs a strategic plan for each business area, which determines each of the marketing objectives/goals, strategies and tactics, that have to be consistent with the overall strategic plan.
Imagine if you can do all these and manage all these in a marketing plan? Systematic, organized, well thought, innovative and captured all your pertinent business information, don't you find having a marketing plan is useful for your business?