According to Wikipedia:
Quote:
The duties of White House Chief of Staff vary greatly from one administration to another, and in fact, there is no legal requirement that the President even fill the position.
[...]
The roles of the Chief of Staff are both managerial and advisory and can include the following
Select key White House staff and supervise them
Structure the White House staff system
Control the flow of people into the Oval Office
Manage the flow of information
Protect the interests of the President
Negotiate with Congress, other members of the executive branch, and extragovernmental political groups to implement the President's agenda
Advise the President on various issues