Calcium is not only important for building and maintaining your bone, but also important for many chemical reactions in your body metabolism and immune, and other chemical reactions at the cell level. Even today, many medical doctors and nutritionists suggest that you drink a lot of milk or take calcium supplements.
Unfortunately, many calcium supplements, such as calcium carbonate, are not very effective because of its low water solubility and its alkalinity. A more readily absorbed form of calcium is calcium acetate which is more expensive of course. Calcium acetate can be created by cooking natural ingredients. If you add a little bit of acetic acid (vinegar) to some beef or pork bones, you will obtain calcium acetate in the broth.
If you are a vegan or being lactose intolerant, you can still obtain tons of calcium from your diet without relying on milk or bones at all. The entire family of "Brassica" green vegetables contain tons of calcium. You can obtain more calcium by consuming these vegetables than by drinking milk. Raw collard green, kale, bok choy, Japanese mustard spinach, ... belong to the brassica family and they all contain approximately the same amount of calcium as milk per cup. If you lightly cook your vegetables, you can actually obtain more calcium than by drinking milk. By eating 2 cups of lightly cooked vegetables, you are effectively eating 4-6 cups of raw vegetables which is equivalent to drinking 4-6 cups of milk.
Eating these vegetables also gives you tons of vitamins A, C, E, and K, plus tons of minerals, such as magnesium. The most important thing from eating these vegetables, of course, is fiber that is essential for a healthy digestive system. Milk does not contain vitamin C and it does not give you dietary fiber. Milk also contains too much saturated fat and lactose. 90% of the world population has some degree of lactose intolerance. Drinking milk can mess up your digestive system in the long-run, if not immediately. Commercial milk may also contain lots of hormones and antibiotics that can cause cancer, digestive problems, etc. Last but not least, milk does not give you the sense of fullness, so that you tend to eat more, possibly leading to obesity.