No proteins are very large molecules comprised of about twenty amino acids, the order in which they are manufactured in the cell will determine what the protein will do.
Because these proteins are used for much more than your muscle growth, which is fueled mostly by what is in whey protein, it is vital to get ALL amino acids into your system to promote proper growth and avoid plateaus... And certain aminos, mainly leucine, isoleucine and valine stimulate muscle growth; as most BCAA's are loaded with these three it is speculated that it will help with muscle growth.
HOWEVER, over the number of studies I have pulled up on it, the results are really mixed on how much they are really able to accomplish in conjuction to a normal healthy diet.