In most cases aax to mp3 conversion is searched by users that would like to convert Audible audiobooks (.aax) to MP3 format so they can play them on any device without any restriction.
There are several programs that advertise that they can create an MP3 file from Audible digital books and during his process also remove the DRM restriction, however whenever they are actually capable of doing so is another matter.
In any case, the resulting quality may be a little worse though and removing the DRM may be a violation of Audible license agreements.