Imagine this scenario: Anarbor puts 4 new songs up on Myspace. They all turn out to be shite. Lots of pissed off fans hear them because there Myspace is the number 1 destination to find the band. Even potential new fans clicking on the page randomly are subjected to the new 'shite' songs.
Now imagine this scenario: Anarbor puts 4 new songs up on Purevolume. They turn out to be good. Anarbor check the plays of the songs to see which ones have gone down the best. A few weeks later, Anarbor puts the better songs on Myspace for the majority of there fans to listen too. Anarbor get lots of plays and everyone is happy.
It's called audience targeting and despite how much you use the site, Purevolume is a very useful tool for bands to use because they can get a general idea of how the songs will do.