Carbohydrates make blood glucose levels go up. Keeping track of how many carbs you eat helps you figure out what your blood glucose levels will be. The more carbs you eat, the higher your blood sugar.
Talking with a nutritionist or dietitian can help determine a daily amount of carbs that will keep your blood sugar from spiking or dropping. Also, if you are on insulin, knowing how many carbs you've eaten is important because it helps you figure out the right dose of insulin to cover the meal.
"Dietary Reference Intakes: Recommended Intakes for Individuals, Macronutrients." Dietary Reference Intakes Table - The Complete Set. 2002. Food and Nutrition Board, Institute of Medicine, National Academies. 29 Nov 2006