While carbohydrates are an essential nutrient, sugar most certainly is not. When you eat sugar (and indeed when you eat carbohydrates albeit to a lesser extent) your insulin levels spike and this is what causes you to store fat. You see, your body can use sugar for energy extremely quickly as there is literally no conversion process required and so your cells get filled with more glucose than they need to perform their function, and when cells have excess glucose the remainder gets stored as fat.