Compact discs or CDs were once used to store music, and you would have to put the CD into the CD player to hear your music. There were even stores dedicated to selling, buying, and trading CDs. You used to keep all your favorite CDs in your car so you could listen to them on the road, and each CD usually only contained a single album, so you’d have to keep lots of CDs on hand if you wanted some aural variety. Obviously MP3 players and smartphones have replaced CDs and CD players, so the lack of CD players in new cars makes sense.