I have different characters because I like variety. I wanted to check out the main story from all starting cities, but since they all converge at lvl 15, it's not a good enough reason to have 3 characters. However, one character is big and burly, so I play him as a tank and getting into monk. Another is a magic user, mostly black mage for now but will eventually level summoner, but I never did like being a healer. Another character is kinda stealthy and likes ranged weapons, so I'm playing him as a bard and will go to musketeer when that releases, and probably thief or ninja later on. One character is genuinely all you NEED for this game, but with 8 character slots per world and up to 40 total (depending on what plan you have) there is no reason why you can't play with more.