is it useful to bring a tent?

No, we don’t think so. Nor cooking utensils. Especially on the Camino Francés, there are enough affordable "albergues" (pilgrim's hostels) and restaurants with a "menú del peregrino" (pilgrim's menu). Also, in some albergues meals are cooked for pilgrims, and in some you can prepare your own meal.
Finally, there are not many places along the Camino Frances where you can just pitch your tent (if allowed locally at all); much land is privately owned.
 
If you decide to take your tent: the Canadian Company of Pilgrims has made a list of  campings in Spain > Downloadable Guides > then click: Camping on the Camino Francés.