It was beneficial for northern Europeans to be lactose tolerant when lactose intolerance was more common in most other places because of the fact of the lactase mutation and natural refrigeration of the climate
Step-by-step explanation:
The most of the places in the world was lactose intolerant as when a child is born, the baby is lactose tolerant and so uptakes as many lactose as needs for it but then while growing up, the enzyme to digest those lactose are lactase which are found to be decreasing in the adult and so adult becomes lactose intolerants.
But in the North Europe and some parts of Central Europe, the climate served to be the natural refrigerator and by the advent of mutation. Northern Europeans became lactose tolerant, as they tend to retain the lactase enzyme. This was assisted with the famine conditions which let the adult have milk as the only option of food and energy.