I'm sure the SD thing works for you but some people need music discovery and open source options are lacking there. When you can set up a radio station backed by RED/Lidarr and not have your account go on ratio watch in hours, then we'll talk..
The discovery algorithm works based on fundamental metadata, additional data pulled from Last.fm (e.g., "related artists" and "popular tracks"), as well as its own acoustic matching algorithm.
This arrangement obviates the SD card, and also any other external syncing (PlexAmp plays live from the server, or has its own scheme for downloading to a local cache).