Because it is impossible to manufacture and traditionally difficult to extract and process from the ground, gold served as a stable peg for coinage and paper money issuance--but no nation operates on a "gold standard" today, as most abandoned it during the 20th century after major powers suspended it periodically during times of costly military adventures during the 19th century.
One of the first acts of the US Congress after the new US Constitution was the Coinage Act of 1792 allowed for the minting of both gold and silver coins at a face value ratio of 15:1.