The main purpose of having crafting up is pretty much this:


Also what Kaitlen listed out are true too, especially the repair part. Even if you don't want to craft for gil or simply having the advantage from crafted gears compare to dungeon/raid loot, having all crafting classes up to level 40 at least (if not maxed) is good thing so you can repair your gear anywhere including inside dungeon (so you won't have to leave the fight out of reason you need to repair), provided that you carry Grade 5 Dark Matters with you.