Before finishing the main story I found it useful to level up/farm gear for my other jobs because I wanted to finish all the class quests before the main story. AND you could replay all the dungeons without having to level them. After the main story they revert back to lvl 70 kinda a bummer... But now I'm slowly using it to lvl another job. So leveling is the reason. Also got the rare items in chests at the end to myself.