Spotify is really sort of a separate internet for music. Anywhere you are, if you hear about a new genre, you can immerse yourself in it within seconds. Imagine doing that back in the days of Tower Records? Even the Tokyo location, or Amoeba Music can't cover the territory like Spotify can. I've tried.
Not having to copy over my playlists to my phone, but just being able to sync, that feature alone is worth the €10 subscription to me.
I discover a song, add it to a playlist from my mac, and I can listen to it next time I leave the house.