Breastmilk is best nutritionally for baby. Thats just science. But sometimes breast isnt possible due to a plethora of issues such as latch issues, nipple confusion, low supply, drying up, smoking/alcohol/drug dependencies, another pregnancy etc. And not to be discounted is the mothers mental health. Sometimes breastfeeding takes a huge toll on a mother and continuing to breastfeed is just too hard. That is a valid reason to switch to formula.