The process of determining public policy is ambiguous and constantly changing. This is because the players in the system change and public's concept of what is essential changes. It is important that this process be vague and adaptable because it were permanently set we would be constantly rewriting the laws to adapt to the current concerns. James Q. Wilson has shown that the mission and cultures of different departments in our government are complex and confusing. It is not always easy to determine what the roles of certain departments are.
Many departments seem to overlap and seem to have unclear missions.
Public policy helps determine what the goals and missions of these departments are. This is going to help the unclarity. Departments are headed by political appointees that are there for a short time. Sometimes it makes it hard to have more than a fleeting impact on policy or on a department. Departmental policies are also set by current public policies so they cannot be set too permanently or they would have to be constantly rewritten.