My guess would be having all of the stuff downloaded and accessible offline.

I just use stremio, if I rewatch something I watched a while ago it’ll download the entire episode/movie again.

Plex I believe allows you to store and host data locally, removing the need to download it all over again, if you are on the same network. You can access Plex remotely but that’ll cost you data again.

I am definitely not a Plex expert so someone else will have to confirm or reject what I said.