The gene could be the genetic recipe for making a medicine such insulin that could be secreted in milk. With this idea, milk from that particular cow would be used as a source of insulin. The insulin would be purified from the milk and diabetics would inject themselves as usual with the purified insulin. Other proteins might be kept in the milk intended for drinking. For example, milk might be made a way to fight cholera.
Cholera is a disease of the human gut.
It's caused when humans drink water contaminated by the cholera bacterium. The best control is obviously clean water. But imagine an additional protection. Imagine if a cow were given the genetic recipe to make a cholera-fighting protein in her milk. In this case, a person who drinks the milk with the anti-cholera protein would be protected against any cholera bacterium in a drink of water.