I don’t remember exactly, but it used to be that you could only stream to mobile devices if you had Plex Pass (I mean, you could just use the mobile browser instead but that is ofc less convenient). Another perk with Plex Pass is that you could download content from the server to watch offline on your device, for example if you’re going traveling. Possibly the built-in subtitle downloader is also a Pass/premium feature.

But otherwise I don’t think it’s necessary. Try it out, all the basic features are available in the free version and spinning it up is super easy. If you decide you like it you can just purchase a lifetime Plex Pass.