By motivating us to satisfy our physical needs, hunger and thirst serve to

Hunger and thirst serve to "maintain homeostasis".

Homeostasis can be characterized as the steady condition of a life form and of its interior condition; as the support or direction of the steady condition, or its balance; or basically as the adjust of bodily capacities. In the case that homeostasis is doing well, life proceeds; if unsuccessful, failure or demise results.