The practice of presidents secretly recording conversations in the oval office was started by President Franklin D. Roosevelt, who was angered by press misquoting him in the run-up to elections in 1940. When Roosevelt died, he left behind the secret listening device in a lamp, which every other president used from time to time all the way up until at least Nixon. The presidents, including JFK and LBJ, were ostensibly recording conversations in order to protect themselves (in the case of Kennedy during important discussions concerning the Bay of Pigs). From the records we have, some presidents apparently used the tapes more often than others, as Eisenhower, for instance, apparently only recorded 5 hours of audio.

Nixon had the most prolific collection of presidential recordings and seemed to become obsessed with installing more and more secret listening devices all over Washington. The recordings of all the presidents were finally exposed during the Nixon watergate scandal. During a live televised interview under oath, in 1971, a white house aide named Alexander Butterfield, who was in charge of installing at least one of the new lines ordered by Nixon, mentioned the secret recordings. According to Butterfield, Nixon's stated goal in making the recordings, which included two hidden microphones in lamps in the oval office, two in the cabinet room, and on all telephone lines in major rooms of the White House, in various office buildings throughout D.C. and even at Camp David, was to "create a more accurate record of events." However many historians point to ego and paranoia as likely explanations.

For further reading on Nixon and his recordings, I highly recommend, Nixon's Shadow: The History of an Image by David Greenberg.

Consequently, after the revelation of secret tapes related to the watergate scandal, congress enacted the Presidential Recordings and Materials Preservation Act (PRMPA) of 1974, which placed Nixon's presidential records into federal custody to prevent their destruction. This is the act that made Nixon's secret recordings publicly available.

The rest of the presidents' recordings, at least those that were not protected for national security reasons, were soon to follow into the public domain, due to various reasons, but also because the recordings were deemed public work created with taxpayer money, and subject later to the Freedom of Information Act.

You can listen to the tapes yourself at the Miller Center, where they are archived by the University of Virginia. https://millercenter.org/the-presidency/secret-white-house-tapes