A few months ago, Spotify introduced Discover Weekly. It's a playlist of music Spotify thinks you'll like, updated every Monday - and it's really, really good. Last week, they introduced Release Radar. Similar concept, only it's new releases on a Friday.
Long story short, once the playlist gets refreshed, all that wonderful music is gone if you didn't save it!
I made these IFTTT (If This Then That) recipes that automatically create an archive playlist of all the music, as it comes out. Select "Discover Weekly (by Spotify)" or "Release Radar (by Spotify)" from the setup, and the rest takes care of itself!