a real world point of view
Why telerecordings were made of Doctor Who Edit
Telerecordings of Doctor Who episodes were made during the 1960s and early 1970s so that episodes could be sold to overseas broadcasters. At that time, many overseas broadcasters didn't possess the technology to broadcast the videotape on which the British Broadcasting Corporation recorded. Thus, Film Recording Clerk Pamela Nash and her staff would oblige overseas clients by making a master 16mm negative of the episode and then burning off prints of that episode as requested.
Because of the expense of the actual medium of videotape, and the then-limited ways in which any video-recorded material could be re-used, it was standard policy to wipe and re-use videotapes. Thus, the telerecordings became the only way in which Doctor Who episodes survived long past their original broadcast for roughly the first decade of the programme's history.
Telerecordings and missing episodes Edit
Between 1972 and 1978, the BBC began to divest itself even of the telerecordings. Some, even most, episodes survived this second purge, surviving in one of two forms: positive and negative. Negatives were effectively the "masters" and tended to be found only within the BBC Enterprises vaults. Positives, or prints, could be found anywhere in the world that the episodes had been bought. For the purposes of restoration, negatives are typically seen as the more desirable, because they were usually more pristine copies.
Because all of the videotape of William Hartnell, Patrick Troughton, and early Jon Pertwee episodes was wiped, the telerecordings became the only surviving video of these episodes. Almost every episode of these three Doctors was telerecorded. Thus an episode only truly went missing once the last remaining telerecorded copy was destroyed by BBC Enterprises, the international sales division of the British Broadcasting Corporation.