It's not necessary that you level a crafter, but it can help you out in the game. Making your own bags and armor (tailor) or making your own food (provisioner) or making your own combat arts (alchemist), etc can save you some coin - unless you are buying the harvestables rather than gathering them yourself, that can get expensive.

As mentioned by others in the thread, there are also quests that require crafting; the manastone heritage quest, for example, requires you have at least level 9 in crafting. There are also quests that are only available to crafters that can give you a leg up on faction grinding.

Best of luck