The Mellotron is an electronic musical instrument invented around 1960 to provide the sounds of violins, cellos, flutes, choirs, horns, pretty much anything, from a keyboard. Given the technology of the day, the reasonable way to do this was with strips of magnetic tape. So the Mellotron uses a strip of magnetic tape, a pinch roller, tape head, pressure pad, and a rewind mechanism for each note on the keyboard.
To our modern day technological sensibilities this cumbersome mechanical contraption seems kludgy as can be, especially you're watching the tape rewind operation, but the fact is that no modern technology keyboard can come close to the quality of presence so characteristic of the Mellotron sound. Why is this? Because the tape playback mechanism is the musical instrument. It matters less what is recorded on the tape.