Milk is usually used to moisten breadcrumbs or bread that is used in the meatloaf. Liquids and starch ( bread/ bread crumbs or Panko) makes what is called a "panade." When you mix it into the ground beef, the panade coats some of the protein and sets it into a gel when cooked. So you get a better meatloaf. Usually I do not use milk, it is either plain water or using onion juice to moisten the bread.