The evidence dairy is a useful food for adult humans is literally in the genes for many people whose ancestors raised cattle. Being lactose intolerant is the human default, but the fact that there are many who can digest lactose proves it has been vital to human survival in some areas.
What something is primarily for is independent of how useful it is to other organisms. Even some birds enjoy drinking milk.
Lactose intolerance is often due to the homogenization process. This process destroys the protein that enables the digestion of milk. Many find they can tolerate raw milk, but not store bought.
Additionally, there are certain proteins labeled A2/A2 that make milk digestible. These are found mainly in the older breeds and less in the mass production breeds (Holstein).
Can you tell I went down a raw milk rabbit hole? π
