Spotify has a similar radio function, it's similarly useful for discovery of new music as you say.When Roon came out, there was nothing available software(app) wise that could approach it's level of simplicity and capability/feature set. This has grown over time, although I have to admit, sometimes bugs do creep in(just like Microsoft and Apple software does). I took the plunge and purchased the software vs. going with a subscription model. To this day, nothing comes close to it. The App is what you as a user, interface with each time you play your music. I also like to research the artists within the app too. Hence it's superior ability. One thing that hasn't been mentioned is Roon Radio, which works amazingly well based on your current song or playlist. Great way to discover music. Before streaming became mainstream, I had ripped all of my music CD's in lossless FLAC and also purchased Hi-Rez music from the various sites. To have access of your personal files is also of benefit to Me.
The cost is substantial, but so is all the gear we purchase/sell and purchase again. This is a one time buy once, cry once affair if you choose to.
If JRiver ever brings in streaming of services to their platform, then Roon would really have to worry. That piece of software is exceptional.
I have no problem accessing my own flac files stored on my NAS via the Wiim app. The interface is a bit meh but it works. That said, I still haven't cancelled my Roon sub yet, which I was always going to do once I found an alternative method of EQ