The benefit of having stuff all on one character is one of convenience. You get access to previously-learned cross-class skills, you possibly have unique dungeon drops for alternate classes, and you get +50% bonus XP if your current class is a lower level than another one. Any crafting classes you have leveled can be used to repair your own gear which is much more gil-efficient.

It should be mentioned that the storylines for the 3 nations are largely similar and become the same once you get your airship pass.

Of course if you have fun playing through the story multiple times in other games and think you would enjoy it here by all means do it. You get multiple character slots as it is.