You know it's useless (I would call it redundant
), but I'm sure there were those that didn't trust the rebounding hammer and wanted a tactile half-cock they could control. So S&W gave them one....
Kind of like some of the current DAO semi-autos (especially the ones with the "G" trigger within the trigger). I seem to remember folks complaining they wanted an actual thumb safety (re the BodyGuard 380) so they could know it was locked up tight.