Blackmail was always the FBI's purpose.

Full text of "J. Edgar Hoover - G-Man In A G-String"

Hoover clung to power for almost 50 years as America's secret police chief, maintaining his position largely through the voluminous files he accumulated on the peccadilloes of prominent politicians: financial, sexual, and criminal.

Anyone remember Filegate?

The White House FBI files controversy of the Clinton Administration, often referred to as Filegate, arose in June 1996 around improper access in 1993 and 1994 to FBI security-clearance documents. Craig Livingstone, director of the White House's Office of Personnel Security, improperly requested, and received from the FBI, background reports concerning several hundred individuals without asking permission. The revelations provoked a strong political and press reaction because many of the files covered White House employees from previous Republican administrations, including top presidential advisors. Under criticism, Livingstone resigned from his position. Allegations were made that senior White House figures, including First Lady Hillary Rodham Clinton, may have requested and read the files for political purposes, and that the First Lady had authorized the hiring of the underqualified Livingstone.