Well - lets say you want to use two (even partially) different sets of crossclass skills - maybe one for Dungeon / Raids and one for open world / single situation. For example you'd use as WHM Eye For Eye in groups but would like to have Virus or some other offensive skill instead in single "mode" so instead of shuffling the skills manually each time you go to battle you can use the macro to switch two sets. Or you can keep the same set but want to set which ones are available at lower dungeons when level synced (typical issue with Protect or Stoneskin which can't be use during level sync if you don't have them in first crossclass slots) - so you can choose which one(s) will be availabe there. Most likely it will anyway not let you to change those during any battle so you need to do so in advance and also wait for action change penalty time to run.