Can some doctor explain to me what protein shakes "do" exactly?

High Protein: Protein is used for many functions in the body, especially build muscle mass. Foods that are typically consumed during the day are not high in proteins, and some people have higher protein needs than others, i.e. special medical conditions, bodybuilders. The shakes provide a high source of protein. Proteins are also considered a good source of calories over fats and carbs for weight loss.