FDR actually began the practice of recording his Oval Office and telephone conversations. During the Vietnam War, LBJ did the same. The benefit of the system was that it gave the Presidents a recorded account of the conversations they were having. This is particularly useful when dealing with issues as complicated as the Vietnam War (or the Second World War in FDR's case). Diplomatic wrangling, congressional deal-making, conversations with department secretaries and chiefs of staff, family matters etc, alot of conversations everyday and having an audio recording (which would be transcribed) allowed the presidents to keep track of everything that wasn't being committed to paper. Nixon actually had the recording system used by LBJ removed when he took office. He had it reinstalled two years into his presidency. I don't know if the existence of the recording systems under FDR and succeeding presidentd was known but I can say that the existence of Nixon's system was a closely held secret that only came to light during the Watergate Scandal. Some of his recordings actually contained references to Watergate and as someone who has used the transcripts of Nixon's conversations with Henry Kissinger concerning the 1973 Coup in Chile, I am incredibly thankful for the existence of the recordings as I'm sure other historians are.