Alts. Maybe you want a separate alt for crafting/gathering for inventory management. Maybe you wanted to do the story a second time. (They're adding ng+ but it's not complete and part of the experience will be lost when not actually leveling/gearing as you go.) Maybe you like different races and don't want to fantasia a lot. Maybe you like roleplaying and want more than one character at at ime. Maybe you like housing and want to have more FC rooms/apartments etc. Or to play with friends on different datacenters.
With the ability to do anything on a single character, and world visit making it easier to play with people on other servers, there's less reason to have multiple characters than in say, World of Warcraft, but still plenty of reasons to do it.
