It allows you to use any USB drive with it and multiple devices can connect simultaneously (it works - I own one). I have a WiFi drive too but it is much better being able to use any of the many portable drives I have with the file hub - and much more cost-effective. The power bank capability with the one I linked is just an added plus.
I actually use this device for playing movies stored on a 1TB external drive. The movies can be played directly from the device without copying locally. Music is the same way but the problem with external devices of any kind and music players is finding a player that can play a selection of multiple tracks. With PowerAmp I can play individual songs without copying them but I don't know a way to do multiple.