Like you said, the only difference is that your body processes carbohydrates first.  However, the consequence of that is that people think they're hungry sooner and then end up eating again.  If they would just stop eating, the body would burn off the stored energy without a problem.  Fats have absolutely no self control, so this results in eating when they don't need to eat.  So instead of eating 1000 calories, they end up eating 5000 or more and counting it as 1000. Edit:  Excess energy is stored as glycogen, not triglycerides.