A Project Plan, like any good document, is a communication tool. Its primary purpose is to formalize and agree on what is being done, when, and at what cost to achieve the project outcomes. Always think of AUDIENCE and PURPOSE when writing anything. A key step early in your plan is to present the plan and get sign-off from the Project Sponsor.
Purpose: To capture, communicate and approve the scope, cost, schedule and other details of a project.
Audience: project sponsor—usually has the final say on decisions related to the project and finances. Also used by the PM extensively.