Please 'splain, my FRiend...MUD
Basically it is a mechansim to order and enforce the performance of a specific public duty. If this is not done then a public explanation is required giving cause(s) why not.
Example, klintoon's disbarment proceeding in Arkansas was initiated/forced under a Writ of Mandamus.
Janet Reno could have been acted upon via a Writ of Mandamus to reveal the LaBella memorandum ... she would have claimed "some priviledge" and the court would have had to decide ... A court decision becomes a "precedence" ... as in, for example, another klintoon ploy ... the so-called "protective right" priviledge which the Supreme Court ruled on ... setting a precedence for further Presidents.
So, when in doubt ... let it snow "Writs of Mandamus."