Although the whole naming convention of RIFF-WAV is highly misleading, its most commonly referenced meaning is to define an MP3-encoded audio file that is stored using WAV file format's headers, thus showing up as an .wav file rather than an .mp3 file.
As the compression level doesn't change, only the file headers change, when converting an MP3 file into an RIFF-WAV file, the only real advantage for the transition is that the tools that require WAV files as their input file format (and don't support MP3 files) can use these files. Most common example of such tool is VirtualDub, an open source video editor.
There are several tools available for MP3-to-RIFF-WAV / RIFF-WAV-to-MP3 conversion operations, but the most popular one must be the open source tool called CDEx.
Note: It should be mentioned here that the term "RIFF-WAV" originally refers to the WAV file format itself, as the WAV conforms to the RIFF format specifications and has been called in various occassions as "RIFF-WAV". The "new meaning" described above has most likely born due the fact that several tools that allow converting MP3s to use WAV headers have described the process (most likely due lack of space in menu fields, so they've shortened the text "MP3 to MP3-encoded RIFF-WAV" to "MP3 to RIFF-WAV" or something..) as converting MP3s to RIFF-WAVs. Thus, the new de facto meaning for the term.
