Don’t hold me to the details, but the hope here is to turn your home into a mini-streaming service just for yourself. If you had TIDAL content, we’d get it from TIDAL, but if you only had it at home, we’d stream it from your home.
There is also the idea of “offline”. Roon would use your local storage on the mobile device to cache songs so they would accessible without internet (from TIDAL and from your home). This could be explicit and/or automatically done based on your listening habits.