Well, no shit Sherlock!
That's exactly the purpose milk is created.
Babies need milk to gain weight and nutrition until it is strong enough to find its own food (I'm not just refering to people here, but all mammals, and anything else which may produce milk for its young).
Once an animal is big enough to eat regular foods in its diet, it no longer needs milk.
As adults, we especially do not need milk.
I don't care how many Got Milk? commercials you see, milk is actually bad for you after a certain point in your childhood.
So, you might say, "Milk has calcium for your bones." Bull hockey... Once you're an adult, your bones no longer absorb calcium. In fact, it's the rest of your body that starts to absorb it. You've heard of calcified arteries/veins, right? Well, your cartilege and muscles start to calcify, too, along with a number of other organs in your body. Just not you bones.
"Milk has vitamin d" you say? Yeah, but that's because it's put in there. Our bodies produce vitamin D all by themselves if we get a good 15 minutes of sunlight exposure a day. Yup, that's all you need to get your required vitamin D. And if you don't like the sun, or work nights, you can get vitamin D in fish oils and eggs, which contail a LOT less fat than milk.
Yes, milk contains quite a bit of fat. Once again, the purpose of milk is for the rapid growth of infant animals (people are animals too).