You don't need milk. A baby needs it's mothers milk for specific purposes. Otherwise, its a sugar laden, fat laden, calorie laden drink.
I love it when I'm bulking or with brownies/cookies, but otherwise, there is no actual evidence that milk is actually "good" for you. They've done studies and found that people that drink milk vs who don't drink milk don't have any noticeable differences in bone injury rates or calcium density. The vitamins/minerals in milk are also easily found in other places.
For most people, its just a easy way of getting calories.