Bread, pasta, beans, potatoes, bran, rice and cereals are carbohydrate-rich foods. Most carbohydrate rich foods have a high starch content. Proteins and fats require more water for digestion than carbohydrates. Carbohydrates are the most common source of energy for most organisms and animals, including humans.
Carbohydrates are not classed as essential nutrients for humans. We could get all our energy from fats and proteins if we had to. However, our brain requires carbohydrates, specifically glucose. Neurons cannot burn fat.