You can do a writ for each type of craft, once a day, per character.
The higher your crafting skill, the better the tier of rewards. At max tier, you often get legendary upgrade materials.
At max craft level, you also begin to earn Master Writs, which provide you with Writ Vouchers upon completion, that you can then use at a specific NPC to purchase exclusive rewards, such as the Ebon style, or several housing related items.
Definitely worth doing, in my opinion.