Strategic planning is a management tool, period. As with any management tool, it is used for one purpose only: to help an organization do a better job - to focus its energy, to ensure that members of the organization are working toward the same goals, to assess and adjust the organization's direction in response to a changing environment. In short, strategic planning is a disciplined effort to produce fundamental decisions and actions that shape and guide what an organization is, what it does, and why it does it, with a focus on the future. (Adapted from Bryson's Strategic Planning in Public and Nonprofit Organizations).
Marketing management is the process of setting marketing goals for an organization (considering internal resources and market opportunities), the planning and execution of activities to meet these goals, and measuring progress toward their achievement.