It’s no longer uncommon for a GM to select specific music to play during a game session. Even in my earliest days of gaming, I’d often put on some CD or other, just to provide some background noise. But more and more, GMs are looking for something along the lines of an actual score for their games. Why is this?
The reason is actually pretty simple, even if many GMs don’t consciously comprehend it (I suspect most of them do, however). It’s the same reason video games feature music, or television shows, or film. Hell, it’s the reason behind opera (plays with music, essentially). Music is not just sound, rhythm, tone, melody, etc. Some of it is, don’t get me wrong. But the best music, the music that lives through generations, is the music that conveys emotion. It could be happiness, frustration, elation, sadness, fear, or any other emotion. And when used properly in any of the media mentioned above, it can kick the emotional punch of the story into overdrive.