There are a couple of reasons: Although not all artists realize it, MP3 is what is known as a "lossy" format. Thus, much of the sound data is removed when MP3 files are created. This results in a file with inferior sound quality to a CD. Vorbis is also a "lossy" format, but uses superior acoustic models to reduce the damage. Thus, music released in Vorbis will sound better than a comparably sized MP3 file.

Also, artists should be concerned about licensing terms for formats. If you decide to sell your music in MP3 format, you are responsible for paying Fraunhofer a percentage of each sale because you are using their patents. Vorbis is patent and license-free, so you will never need to pay anyone in order to sell, give away, or stream your own music.