EMI Music announced DRM-free music downloads through Apple's iTunes earlier this week. They mentioned "premium" music files, with a bitrate of 256 kbps, instead of the standard 128 kbps, for thirty cents more. Slate answers the question: So what?
Listeners don't need all the data on a CD, since much of it is imperceptible to the human ear. Sound compression takes advantage of this fact by removing all that extra information. For starters, codecs throw out frequencies outside the range of human hearing - roughly 20Hz to 20,000 Hz. But that only accounts for a small amount of savings. To save even more space, the codecs also scrap frequencies that would be audible on their own but become virtually imperceptible in the presence of other sounds, like a booming bass. A well-designed codec will only get rid of stuff you wouldn't notice in the original record; that's why the codec you use to compress a file can be more important than its bitrate.