Preservation is extremely relevant for streaming services, because you don’t own anything. Sure, that movie or show is there today, but the streaming service could decide to take it down tomorrow, and then it’s gone. Maybe forever. Corporations have no interest in preservation, unless it makes them money. And sometimes, oddly, not even then.
There can also be weird issues with copyrights or something like that. Take the example of old Beavis & Butt-Head episodes. MTV had to cut out the music videos, because they no longer held the rights to show them anymore. There are episodes of Mystery Science Theater 3000 like this. They had permission to show that movie at the time the episode was made, but they don’t hold those rights anymore, so they can’t offer that episode for streaming.
It’s important if they share the file, too.