Like Bruce Lee ascending the proverbial Kung Fu levels of excellence in the pagoda – in his yellow jumpsuit – athletes ascend a series of levels of understanding of the body.
One of those, an important one, is providing enough protein to make sure all those zillions of little torn muscle fibers have the proper nutrients to rebuild. Lots and lots of protein. Proteins are about 50% of the dry-weight of most cells.
Since many people are under the delusion that meat is bad for you, and since meat is traditionally the primary source for humans to ingest protein, there can be a great barrier to achieving desired gains.
Whey is thought to be a better form of supplemental protein than soy and some of the cheaper types. This understanding will surely be overturned as some point, if not already. “Whey” is not the point.
I can think of no direct stand alone benefits of drinking a whey protein shake. It can however serve as a very good supplement to someone who is working out hard, is focused on building muscle (vs. strictly loosing weight) and who is paying close attention to caloric intake (vs. say eating a half pound of peanut butter for dietary protein).
That is my simple understanding. I look forward to an updated one.