I guess two standard answers would be '...because we can...' and '...just to be different...'. Granted, neither of those are very good answers.

I first did it because someone on The High Road said it could not be done. Also, I have over 1K of factory .38 Super ammo that I consider as backup to my .38 Special and .357 Magnum stash. I don't carry .38 Super in .38 Special or .357 Magnum revolvers, but it's nice to know I could if the ammo well ever ran dry.