"worth" is subjective. A loss of culture is a loss of culture, whether you see value in that lost culture or not. It's a part of our society's history, of which many people usually contributed to across a wide variety of skills.

How do you know if they're worth preserving or not if you can't see them?

And "worth preserving" is a term that can be seen from a myriad of perspectives. Whether it's good or not is only one factor.

A film or television episode might be worth preserving because:

* it's an early work or appearance by someone who would go on to be significant

* it's a product of its era displaying the context, fashions, language, politics, events and attitudes of the time

* it's actually really bad, but demonstrates what bad film-making looked like in that time

* it's a cherished childhood memory of someone who mentions that in their autobiography, and so scholars may wish to have access.

Probably loads of other reasons why something is worth preserving beyond a basic question of "is it any good?".

The BBC destroyed and disposed of a large number of episodes of Doctor Who from the 1960s, and a lot of time and effort has been put into recovering and restoring copies of these because they are a part of the ongoing narrative of the evolution of British television.