If you convert to mp3s (for purposes of convenient listening, not storage, as mp3s are a 'lossy' format) You can convert to 'mono' and thus save half the digital storage space.

Plainly stated, mp3s typically have two tracks, but there is such a thing as a 'mono' mp3 file which consists of a single track but it is sent to each ear. Since your music is mono anyway there is nothing to be gained by converting in stereo format.