The answer to your question is in the question itself: Not everyone owns every song they listen to, or would listen to if they could.
New songs and artists come out. You want to discover new music. You want to listen to a song or album maybe once or twice, so it's not worth purchasing. You've got friends or family over, and they want to listen to their music. Your tastes in music change, and the albums you purchased previously don't get played much anymore. You mostly listen to a few genres, but listening to classical helps you study or read. You want to potentially play any of 75 million songs, but you couldn't find a spare $75 million between the couch cushions.