“Best of” albums were once a huge source of revenue for record labels, and a great way for music lovers to get an overview of a favorite band. Not so much anymore.

In the 1950s and 1960s, there was a market for singles and a market for albums, and they were almost completely separate and aimed at different audiences. Singles – 45s – were cheap and directed at teenagers. Albums cost more, and so they were marketed to adults. That creates some disparity in the music charts of that era. For example Elvis Presley scored #1 hit after #1 hit, while album sales were dominated by Harry Belafonte and Broadway cast albums. Sure, Presley and other rock acts released albums, but they didn’t necessarily contain songs released as singles, which were often released as standalone items.

One of the few acts of the time that sold both a lot of singles and a lot of albums was Johnny Mathis. In 1958, Columbia Records decided to take all of the crooner’s singles-only releases and put them on one LP, Johnny’s Greatest Hits. Those who already owned the songs individually were enticed to buy with the inclusion of four new Mathis songs. The gambit worked: Johnny’s Greatest Hits sold five million copies. It was the first-ever “greatest hits” album.

These albums aren’t purely the result of crass marketing. “Best of” collections are a way for new fans to sample an artist, a gateway to discovery. In fact, a quarter of the 200 bestselling albums of all time are greatest hits albums, including collections by Elton John, Journey, and the Steve Miller Band. For many artists, the bestselling release of their careers has been their greatest hits album. These older greatest hits albums by the major acts of the ‘70s and ‘80s continue to sell, still moving a few hundred copies a week each. Until Michael Jackson’s death in 2009 led to a few million new sales for Thriller, the bestselling album of all time, with 29 million copies, was Their Greatest Hits (1971-1975) by the Eagles.

Greatest hits albums are not the perennial cash-cows they once were. That’s because there are fewer widely popular, blockbuster acts, and overall album sales are down from what they were a decade ago. Kelly Clarkson, one of today’s most popular pop singers has had more than a dozen big hits. Her greatest hits album, released in 2012, has sold less than 500,000 copies. However, the new song on the album “Catch My Breath” was a top 20 hit—thousands of Clarkson’s fans bought the single, a la carte, on iTunes.