Why Have a Dedicated Music Device?

In the last year or so I've read about many people moving from streaming services, like Apple Music and Spotify, to their own music library. To support these local libraries, many seem to be getting themselves a music player, such as the Fiio Echo Mini. While moving to a local library is something that I've thought about many times1, I don't understand why people are buying these little music players. The big selling points generally seem to be: Bluetooth connectivity so...