Harmonix Music Systems redefined the peripheral-based rhythm genre with Rock Band in 2007. Five years, two numbered sequels and a few notable band-specific spin-offs later, their fans have a huge music library at their disposal. Thanks to smart thinking at the outset, almost all of the material from each successive game release has been playable within the Rock Band ecosystem (The Beatles Rock Band notwithstanding). There's somewhere in the neighborhood of three thousand songs in the Rock Band catalog if you count the community-authored tracks available on the Rock Band Network store.
The problem is, no one wants to buy peripheral-based games anymore. The novelty's over and isn't coming back anytime soon. Despite this, Harmonix has been keeping Rock Band fresh by releasing new songs and albums each week for players to download. This is great for hardcore fans, but not everyone wants to go through the hassle of getting the band back together because they want to play a few new tracks. That's where Rock Band Blitz comes in.