In that case, perhaps I have a better idea of when it could be useful.
If the CD is detected, the In-Game music would be set to on by default
(and greyed out if the CD isn't). Next time you run Re-Volt again tho,
the CD Check notification will show to notify you that you haven't
inserted the CD.
Related to the grey-out of In-game music, I expect that MP3 support
has it's own option as some people (like me) wants both Redbook music
on stock tracks and MP3 music in custom tracks. :)