Plain potatoes, rice, pasta, and many types of bread are fat-free. The fat comes along with the butter, cream, and cheese that you add to them. Of all the nutrients, carbohydrates are the best source of energy. It takes a while for the energy from protein and fat to get into your system. Your body can use the energy from carbohydrates right away.
Carbohydrates contain lots of in vitamins.
In other words, it helps you have regular bowel movements. Not all carbohydrates provide these benefits. Plain sugar, honey, corn syrup, and molasses don't add anything to your diet but yummy flavor and calories. In fact, they actually make you hungrier and weaker than before you ate them. You eat a candy bar.