I just looked this up last night and I quote
"WAV format is necessary for editing sound files. Technically, MP3 cannot be edited to remove defects or add effects, it must first be converted to WAV format. A WAV editor is needed to alter the WAV file. Some programs include a WAV editor in the program."
taken from http://www.mp3-converter.com/faq/wav.htm
that is why programs like audacity switch the mp3 to edit and then have to encode it back to mp3.
As far as I understand it, If I'm wrong I'm curious for the answer myself.