eli5: why do some videos have to rebuffer parts already watched when played back from the beginning?

Question

eli5: why do some videos have to rebuffer parts already watched when played back from the beginning?

In: 18

Answer 1

The idea of a buffer is that it’s a sliding window, so to speak. It keeps in memory the parts of the movie shortly before and after the spot you’re currently at. So as the movie plays, the buffer discards earlier parts.

If it didn’t do that, the movie player would consume a ton of memory.