Sometimes you have a .wav or .aiff file, and you want it in .mp3 format. SlowGold, for instance, exports audio in these formats if you choose to save a slowed-down song or a segment of such a song. SlowGold does not, however, write mp3 files because of the prohibitive license costs for doing this directly (it reads and plays mp3s perfectly well, of course).
Apple, however, is not constrained the way we are when it comes to mp3 writing, and iTunes can convert between formats, although the functionality is somewhat hidden.
The current instructions for the conversions are at https://support.apple.com/en-us/HT204310. Enjoy!