Some people have a lot of music in mp3 format, but do not have the original CDs (*cough*). Others have the CDs, but spent months ripping and encoding all of them into mp3 format and don't want to go through the trouble again. Such people are often tempted to take their mp3s, decompress them into WAVs, and re-encode them into Ogg Vorbis files. Some have even gone so far as to create tools to automate this process.
If you care about sound quality, you should never, ever do this. Ogg Vorbis uses similar but different techniques to remove information, and by transcoding, you lose information twice. Similar to faxing a photocopy of a fax, the "transcoded" ogg will always sound worse than even the original mp3.
Besides, for most users this isn't necessary, as almost every player that supports oggs supports mp3s as well, and your mp3 collection can peacefully co-exist with your growing ogg collection. In fact, the only compelling reason to get rid of existing mp3s is ethical, not technical: either your mp3s were illegal copies or perhaps because you don't want to support a patented format.
However, if your mp3s are of exceedingly high quality (say, 256 kbps or higher), then you probably haven't incurred enough quality loss to worry about. They could probably be converted to Ogg Vorbis (especially at lower bitrates) with little additional loss of quality.