To start, people obviously know milk is good to drink since it's what we produce to feed our young. It takes no great leap of logic to realize that the milk from any mammal is probably about the same in content. And so, since human females can't produce enough milk to commercially profit on it or, in earlier times like I'm talking about, live off of it, a large yet easy to care for mammal was enlisted to do the job. Eventually these "cows" became domesticated and are still used widely today.
Obviously.
“I’m a stupid fatty and I love to play with my Easy Bake oven!â€