Final answer:
A mission statement is the management tool that describes why a department or organization exists and its key functions, offering a clear guide for its purpose and goals.
Step-by-step explanation:
The management tool that best describes why a department or organization exists and its key functions is a Mission statement. The mission statement outlines the purpose, goals, and key responsibilities of an organization. It serves as a foundation for aligning the company's strategies and decisions.
A Strategic plan, on the other hand, details long-term goals and the actions needed to achieve them. A Vision statement describes the future position of the company, where it aspires to be, inspiring members towards this image of the future. Policy and procedure are the specific guidelines and methods for enacting the organization’s operational processes and maintaining consistency.