In case you don't remember the halcyon days of plastic instrument parties, Rock Band is a fairly simple rhythm game made interesting by the fact that you play it with plastic versions of guitars, drums and keyboards. The conceit was that you and your friends were the titular Rock Band, and your basement was a stage.
The game itself is pretty irrelevant at that point. Sure, if you're better at timing your button presses you'll get a better score and the song will sound better, but you're not really noticing any of that once you reach a certain threshold of quality, you forget all that. You're on stage, you're playing the song. You are, for a brief shining second, role-playing as a rock band. That roleplay element is something that most video games can't really get us to participate in.