In a nutshell, if your app isn’t able to make a direct connection to your Plex Media Server when you’re away from home, we can act as sort of a middle man and “relay” the stream from your server to your app. To accomplish this, your Plex Media Server establishes a secure connection to one of our Relay servers. Your app then also connects securely to the same Relay server and accesses the stream from your Plex Media Server. (In technical terms, the content is tunneled through.)

So, your Plex Media Server basically “relays” the media stream through our server so that your app can access it since the app can’t connect with your server directly.

Source: https://support.plex.tv/articles/216766168-accessing-a-server-through-relay/

It's not a requirement to stream and it's sort of dumb they are lumping this relay service as a part of the remote streaming. Remote streaming should be allowed for free - if you are not a subscriber. The relay should just be a paid service, which makes sense. But if it's a direct connection to my server, it should be free.

That being said, I understand how Plex may have built some technical debt into this relay system. It might be hard for them to decouple the relay from the remote streaming. What they should have done is:

We are removing the relay service as a free service, but you can still do remote streaming with a direct connection.

And they should have built their architecture in a way that's easy to decouple the two services.