I think if you replace "Greatest Hits" with "Top Sellers", you get a better sense of it. Fans of a band always have personal favourites that never got a lot of, (if any), airplay. These compilation albums are a chance to generate some high-profit sales without incurring the expense of recording and producing new material. It's a crass, commercial, ploy, but from a less-than-fervent fan's point of view, it can pull together enough of a band's recognisable songs to justify buying a disk that might not otherwise be in their collection. The Crass part of it is that, unless the band owns all of the rights to the music, the dollars from compilation sales go to the record company, not the band.