The whey provides a quick hit of aminos to boost protein synthesis, while the casein provides a slower source of aminos to slow down protein breakdown, plus you get
slightly different amino acid profiles.
The human body is perfectly capable of forming complete proteins from different foods
with different amino acid profiles, provided the combination provides all Essential Amino Acids.
Bone broth and collagen both have
different amino acid profiles than muscle meats.
But in general, plant proteins are less available, have
a different amino acid profile and are accompanied by numerous other factors that alter metabolism.
Why Eating a Variety of Protein Sources Matters - Because protein is comprised of up to 20 different amino acids, each source contains
a different amino acid profile.
Because protein comprises up to 20 different amino acids, each source contains
a different amino acid profile (which either has some or all of those aminos in different proportions).
It also provides access to
different amino acid profiles.