Michael D Mirne writes …
This is actually a very good question and unfortunately, the previous answers all avoided the essence of the question.
We know that even studio recordings that do not have overdubs still require some isolation of tracks. Otherwise, the drum mics would pick up the guitar amps, and the guitar mics would pick up the sound of the drums. Why does that matter? It matters because if you have “cross-contamination” between the tracks, the music will be difficult to mix. The piano, bass, guitar, and drums should all be on separate tracks of the tape, even if it is a semi-live recording.
In the early days of multitrack recording, the musicians didn’t use headphones, but rather speakers. This led to the obvious problems, so by 1967, pretty much everyone switched to using headphones. The Beatles used headphones in the Get Back sessions, even if you didn’t see that in the movie.