OH I see. Yes, MP3's are smaller than CD audio files but there's a loss of quality. It's hardly noticeable to most people and usually I can't tell at all. If you have a CD with MP3 files you can get maybe 10 hours or so of high quality audio on one disk. Some people really want the full range of audio and will pay extra to buy multiple disks.
Music is rarely sold this way on CD. Most of the time MP3 disks are spoken word works.