The way they impliment it is pretty bad. Basically you have to add a local music folder on your PC, then you have to use the search tool to find it (on PC) and add it to a playlist. Then you have to go onto your mobile while on the same network as the PC, and download the playlist.

Spotify doesn't store your song at all on their servers (unlike google) and if you want to re-download the files on a new device, the original pc that had it has to still be there or else you have to redo the whole thing.