Crafting is the one I've found to be the most useful. Even if, like me, you don't mind getting all your own materials, if you haven't leveled a certain crafting job and need items they make, then you need to buy the items for recipes in that case... Or take the time to level that crafting job before you can progress with your current one. It can be a time and coin consuming rabbit hole.

That said, it can be avoided by leveling multiple crafting jobs at once and gathering for yourself. ...Then your biggest problem becomes, not Gil, but space.