Glucose is the body’s chief source of fuel.  In the human body, "cortisol" is needed to stimulate a process called "gluconeogenesis" in which the liver turns fat and other substances into glucose.  Without cortisol, the body will eventually run out of glucose (fuel), get sick and die. 

People normally produce more cortisol when they are sick.  When you have CAH, you make up for this by giving additional cortisol medication ("stress doses").  Thus, monitoring blood glucose levels during illness is the doctor’s way of making sure your child is getting enough additional cortisol.