Well are you pirating stuff? I've been using Plex for years now and so its mostly just what I know but when I first started using it the main things it did were a) handle codec issues (transcoding on the fly when needed) b) pulling metadata and images for movies and shows based on title and year in the file name. C) allow remote users (friends, family, etc) access to your library via the app, which works out to being very similar to a regular streaming service from their perspective. Basically nothing else did that and so it had a big leg up over XBMC and other apps folks used back then.

If you're purchasing everything through iTunes (do people still purchase stuff through whatever is "iTunes" now? I guess I don't know that either) I assume its handling transcoding/different device playback and delivering all the metadata for you.

Also once Plex pulls metadata down you're right that it doesn't store it with the file but AFAIK its not constantly hitting the net to pull that info down - it keeps a local cache.

I will say at this point - I wouldn't bother switching to Plex and look for an alternative like Infuse. The company is clearly under pressure to monetize beyond the Plexpass subscription you can buy. They've been steadily adding crap no one wants and automatically jamming it into the home screen of the app where you then have to go turn it off. Its just a matter of time before they cross a line somewhere and people jump ship. When that happens I imagine some of the open source alternatives (Jellyfin) will see a huge influx of development. I haven't switched just because I don't want to be hassled with figuring out a new system.