If I understand correctly, it was originally implemented when they made it so you could use ssl to access your media without any configuration or cost: https://www.plex.tv/blog/its-not-easy-being-green-secure-communication-arrives/

I also think you can watch locally without logging in, but, it’s a less than ideal way of doing it: https://www.plexopedia.com/plex-media-server/general/plex-no-internet/

Unfortunately, the biggest red flag about Plex is that they now offer their own streaming media. That means they’re in bed with media companies which is at odds with the goals and needs of the original fans and users of Plex servers.

When I saw the first slow steps Plex’s encruddification, I was relieved to find out Jellyfin exists. I wish it had more features, but it’s being actively developed and totally usable already. Also, I’m not a fan of the name, but that’s a stupid thing to complain about.