The reason is so as to be able to play files encoded with Ogg Vorbis and AAC (LC-AAC, not -- except for Toshiba Gigabeat, HE-AAC); with the former, file size can be at least 10% smaller for comparable quality. And with Nero's freeware LC-AAC encoder, almost as small a file is possible -- and those will play on umodded iPods. For the former, look for OggDropXPD at rarewares.org (aotuv build) and the latter is most easily done with Stanley Huang's MediaCoder. Older versions of the latter will work in WINE in Linux.