The restriction of fat intake in dieting has been a long standing principle used for weight loss and weight control. Biochemically speaking, fats are the most precious energy reserve in the body. This is due to the fact that it has the highest energy (or caloric) value. Aside for being more efficient in energy generation, fat is stored on the body without water, so it does not weigh as much as protein (muscle). So a human can have more energy stores and carry less weight with fat than with any other source of energy.
The interesting thing about fat is that the body can actually build fat out of excessive consumption of carbohydrates or protein. Once carbohydrates or protein are converted to fat, they can not be converted back. So it can be very difficult to loose the fat once it is stored. This is why some ‘non fat' diets are not effective. If you eat too much protein or carbohydrate, they will fill the reserves initially and then the body will convert the extra calories into fat!