Protein is the building block of muscle, and without enough of it, your body won't build muscle. Of course, you should try to get most of your daily protein from regular foods. However, there are a few times of the day where protein shakes would be better. Out of the many protein sources out there, whey protein is the ultimate.
Whey protein is very, very important for bodybuilders, Since athletes and bodybuilders work out often, protein levels become depleted. All protein that is lost within the body must be replaced and protein must be present in order for new tissue to be constructed. Protein is the building block of your whole body - muscles, tissues, hair, nails, etc - all made up of protein. If you do not have enough protein in your body you can NOT build more muscle mass.