heres a direct quote from my weight lifting book:
"what you need to know is that your body can use protein to build, maintain, and repair muscle tissue. As your progress, your will need quick and easy means of getting sufficient quantities of protein. The best way to do this is by drinking a high quality whey protein shake. Whey protein has long been a staple for increasing lean muscle tissue because of its broad spectrum of essential and nonessential amino acids."