Originally Posted by Master Ozzy:
“I never understand the point of Greatest Hits albums. They're just a cheap money maker for record labels and usually artists don't actually want to do them, it's the label that pushes them. All of the songs are already out there, so what's the point of putting them together on one album? If people want to listen to the songs then they can as they're all already available.”

I think they appeal to casual fans of artists, i.e. people who like an artist's singles but wouldn't buy a full album.