Whey protein is considered a complete protein and contains all 9 essential amino acids and is low in lactose content. People commonly use it as supplementation, alongside resistance exercise, to help improve muscle protein synthesis and promote the growth of lean tissue mass.
Whey digests very rapidly. In fewer than 30 minutes it can fast-track a good portion of its aminos to your muscles, and that rapid delivery of amino acids to muscle cells has been shown to be important for pushing muscle growth.