THE BENEFITS OF CARBOHYDRATES.....

Carbs Are a Source of Easily Obtained Energy

The principal benefit of carbs is easily-obtained energy in the form of glucose. When we eat foods containing carbohydrate, digestive enzymes in the mouth (saliva), stomach and intestine rapidly break it down into simple sugars and ultimately glucose. The glucose is then absorbed through the walls of the intestine into the bloodstream, and then either distributed to cells and muscles with the help of insulin for immediate use, or retained as an energy reserve in the liver and muscles in the form of glycogen, or stored as body fat. Although protein gives us glucose too, it takes much longer. Furthermore, some cell tissues (eg. in the brain) depend upon glucose from carbohydrate, which is why one of the side-effects of low-carb diets is a reduction in brain function. The ease and speed with which the body can convert glucose from simple carbs into energy, is illustrated by the fact that diabetics suffering an episode of hypoglycemia (very low blood glucose levels) can make an almost instant recovery after eating a couple of cubes of sugar, or equivalent.

Carbs Benefit Muscle Tissue

When our body needs energy it first looks for glucose from carbohydrates. If insufficient carb glucose is available from food (eg. because your diet is very low carb, or because you have metabolic disorders preventing normal use of available glucose), the body helps itself to glucose stored as glycogen in fatty tissue. If more energy is needed, the body then burns protein tissue in the muscles. In short, the availability of carbohydrate energy prevents the body from breaking down muscle tissue for fuel.

Very Complex Carbohydrates (Fiber) Benefits Digestion

Dietary fiber is classified into 2 types: insoluble and soluble. Both types of fiber are composed of dense indigestible polysaccharide carbs whose structure cannot be converted into glucose by human digestive enzymes. Paradoxically, the indigestibility of fiber makes it a very healthy addition to our daily diet.
Insoluble fiber (eg. cellulose, a few hemicelluloses and lignin in plants and whole grains) benefits digestion by stimulating peristalsis - the muscle movements that propel food along the colon. Being bulky, the fiber allows the colonic muscles to get a better "grip". Point is, if food moves faster through the large intestine, the risk of digestive disorders (eg. constipation, diverticulitis, even colon cancer) is reduced. 

Soluble fiber (eg. pectin from apples, beta-glucans from oats) appears to reduce cholesterol levels and may reduce the risk of atherosclerotic heart disease.
Both insoluble and soluble fiber helps to make us feel full after a meal, and keeps us full for longer. This extra satiety-effect reduces the risk of overeating and consequent excess calorie-intake, which benefits obesity and weight management.

Very Complex Carbohydrate Benefits Blood Sugar Levels

Sometimes, eating too many high GI carbs (the ones that are very quickly broken down into glucose in the stomach) can trigger a very rapid rise in blood sugar. This "sugar-spike" can cause food cravings, appetite swings and, over time, impaired glucose tolerance or insulin insensitivity. However, the presence of dietary fiber in the digestive tract can help to slow down this conversion of carbs to glucose. Result? Blood sugar levels rise at a more normal speed thus avoiding the above health problems.

Other Benefits of Carbohydrates

Certain carbs benefit digestion by providing nutrients for healthy bacteria in the gut. In addition, carbs may benefit calcium levels by increasing calcium absorption from food.