I think you mean the Secret Service started out as the Pinkerton Detective Agency. They became part of the government due in part of protecting the president and investigation into monetary related crime.

The FBI came in early 20th century as a need to address more conventional crime that crossed state lines and made it difficult for states to investigate/prosecute (organized crime, kidnapping, bootlegging). While you could say it had a valid reason to exist, J. Edgar Hoover morphed it into something entirely different.

Edit: To clarify my hastily typed up message from my phone, the Pinkertons were government contractors and precursor to the Secret Service, but not the same entity. Secret Service did not officially protect the president until the assassination of President McKinley in 1901.

Neither one is true.

The Secret Service was initially chartered solely as an anti-counterfeiting task force. The Pinkertons never became part of the government; they were just a handy private army to be deployed by people who'd obtained the good will of the government.

Mea Culpa. Should have double checked before typing that out :)

Didn't mean to say they were the same thing, just the precursor of the Secret Service and did much of the same work in a contractual role.

Anti-Pinkerton Act passed in 1893, denied anyone working for the Pinkertons or similar organizations to work of the United States government or the District of Columbia.

That's not true.