According to the USDA website:
Why is it important to eat/drink dairy?
Consuming dairy products provides health benefits — especially building and maintaining strong bones. Foods in the Dairy Group provide nutrients that are vital for health and maintenance of your body. These nutrients include calcium, potassium, vitamin D, and protein.
How much food from the Dairy Group is needed daily?
About 90% of Americans do not get enough dairy, therefore most individuals would benefit by increasing intake of fat-free or low-fat dairy, whether from milk (including lactose-free milk), yogurt, and cheese, or from fortified soy milk or yogurt. The amount of dairy foods you need each day depends on your age and can vary between 1 ½ to 2 cups for toddlers, 2 ½ cups for children under 10 and 3 cups for older children through adults.
The Dairy Group provides many nutrients including calcium, phosphorus, vitamin A, vitamin D (in products fortified with vitamin D), riboflavin, vitamin B12, protein, potassium, zinc, choline, magnesium, and selenium.
Calcium is used for building bones and teeth and in maintaining bone strength as you grow older. Dairy products are the main source of calcium in American diets.
Many people do not get enough potassium. Diets rich in potassium may help to maintain healthy blood pressure. Dairy products, especially dairy milk and yogurt, and fortified soy milk, provide potassium.