Well, Spinel is the warper, which allows you to warp to many maps including some boss maps and monster maps which makes it more convenient for players.
Duey is the pvp guy who sends you to the pvp map so it would make it way easier to just click trade and click on duey instead of walking around hopeless looking for him.
Cody is also eescential if your like training on a map which is hard to get to, it would be easier for players to find a safe spot and click trade and job change and resume there training on that map.