Athletes, regular hard-working folks, and vegetarians who eat dairy can best benefit from the protein in whey as opposed to other forms of protein. Whey protein's amino acid makeup, branched chain amino acids (BCCAs), are metabolized directly into muscle tissue and are the first ones used when a muscle cell is taxed in exercise. These "lean muscle tissues" rely on BCCAs to begin the rapid replenishing and rebuilding of the cells.
Whey protein is also easily digested and gets into the bloodstream much more rapidly than proteins found in meats and even tofu. Tofu is great, mind you, but you'd have to eat several blocks a day to get the equivalent benefit from whey protein. This also assumes you are looking to build lean muscle mass and lose fat on especially arduous exercise program. Otherwise, any experienced vegetarian knows how to get all the protein they need. Carnivores too.
Lastly, in defense of whey, it is the abundance of the amino acid leucine in whey that makes it a no brainer addition to your program. Leucine promotes muscle protein synthesis and growth. Glutathione is also found in whey at high levels and is an anti-oxidant that plays an important role in the immune system: heavy exercise can deplete the body of Glutathione.
There is plenty of information to be found on the benefits of whey in your routine, though there are also detractors. I have found it to be very useful in strength training and weight management.
The picture shows my favorite brand, though there are plenty. You can buy it in bulk at some Whole Foods, Vitamin Cottages, etc.
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