Why do we need macronutrients?

Carbohydrates: Carbs do many functions for our body. We get energy to do daily tasks from carbs. Our body looks for carbs to burn when we start to do an activity. Carbs also protect our muscles. They do this by providing our needed energy. If we do not have any carbohydrates, our body goes to our fats and after running on our fats for too long, we run out of those energy sources. When this happens, our body goes for the next source of energy, proteins. Proteins are found in our muscles and when our body goes for the proteins for energy, our body is attacking our muscles. Our body starts to eat away at the muscles. Without carbs, our body would lack a large portion of our bodies needed energy.

Fats: Fats do many functions for our body, many of which are similar to what carbohydrates do for our body. Fats are our long term energy that we use when we run out of carbs. But that is not all fats do. Fats also support cell growth. Fats cause for the hormone leptin to go and start burning fats. When this happens, TNF (Tumor Necrosis Factor) starts to try to control the hormone. When TNF is at work, it also can help heal and repair cells.
While fats are doing this, they simultaneously are providing insulation for the body. When a controlled level of fat is maintained, our body uses it as a thermal layer. This protects our body from cold conditions. It, in that state, has the capacity to control our temperature.
Also while in this layer state, they create a layer of padding to protect our organs from injury. On top of providing all of this, fats help absorb the vitamins our body needs. Vitamins are stored in fat for when we need them. They are truly beneficial when in the right amounts.

Proteins: Proteins are a key component to most of our bodies structures. They are found in bones, muscles, hormones and many other places. Proteins make up 45% of the human bodies structures. In muscles, protein make up 20% of the make up of the muscle. Since protein is found in this large portion of the body, they also are used to repair what is damaged. Protein also helps to function what it is found in. For example, proteins help to contract the muscles in which they are in. Although protein is very beneficial, do not try to take it in excess. Extra protein gets converted into fat and when we already get all of the fat we need daily from our regular diet, adding to the fat count is unhealthy and can become a health risk if not controlled.

Sources:
http://www3.georgetown.edu/admin/auxiliarysrv/dining/nutrition/protein.html
http://www.dummies.com/how-to/content/carbohydrates-do-more-than-make-energy-for-your-bo.html
http://www.americanheart.org/presenter.jhtml?identifier=3045789