Signs of change in osmolality, stomach stretch, and thirst are monitored in the urine and the feces of a patient. These things are specifically ordered by doctors so that they would know the body's water balance and its ability to produce and concentrate urine.
Aside from these, getting a sample of a person's urine and feces would help doctors to investigate if there is low sodium levels within the body, there are toxins, and to determine what kinds of treatments can and cannot be done to the patient.