Infant formula is the next best food for baby after breast milk.
It's actually probably in about fourth place, after breast, breast, and breast. It is made by heat treating cow milk proteins to make them more digestible. Unprocessed whole milk forms large curds in the stomach that are very hard for baby to digest. That way baby gets a complete diet with all required vitamins and minerals. It's also a lot cheaper, and lighter to lug home from the grocery.