That is required so that you can easily tell whether a high proportion of the calories are from fat. Usually the label also tells you what percentage of calories in a serving of the food come from fat, so you can easily compare between similar foods if you are making a choice. A lot of nutritional guidelines (the one suggested by the Mayo Clinic for instance) suggest that you limit your total fat calories to less than about 30-35% of your diet. So in an 1800 calorie a day diet, that would be less than about 70 grams of fat. If you were following those guidelines, you could add up the total fat grams, or instead avoid foods that were much higher than 35% in their fat percentages.
People who believe that saturated fat is connected with blood cholesterol levels and heart disease also want to limit their total consumption of saturated fat, so that's why that's usually listed also.
Here's an explanation from the Mayo Clinic: Dietary fats