Varying opinions here, but...

- craft items - (low level) for people to buy to use
- craft items - some people buy to turn in for various rewards... (i.e. too lazy to craft themselves, so they buy off the market board)
- craft items - well, obviously to level up to where you can make the higher tier items.
- Ability to repair gear yourself; relative to the craft and the gear type.

Personally, I went with fishing first and use that to make some money since I'm currently in the stage of acquiring materia for Animus to Novus gear. For the crafting I've done so far outside of a gathering job, I've been able to sell most of what I've made.

Other veteran crafters can chime in with their opinions, but you certainly don't have to level a crafting job.