Every cell in the body requires protein. Your body uses it for growth, tissue repair, creating hormones, enzymes, and energy. Protein is an important component of bones, muscle, cartilage, skin, and blood.
Protein is a "macronutrient," meaning the body requires relatively large amounts of it. Fat and carbohydrates are also macronutrients. Oppositely, vitamins and minerals are micronutrients because they are needed in only small quantities. While the body needs a large amount of protein, unlike fat and carbohydrates, the body cannot store protein. Thus, the body can't withdraw more protein from a reserve when it needs more.
Because protein is essential for growth, it's particularly important for those in growth periods of their lives. People in growth periods include children, teens, and pregnant women. Protein also helps preserve lean muscle mass and provides energy when carbohydrates are not available.