Breast feeding is not always possible, some women are unable to nurse for various reasons and in some rare cases babies can have an allergic reaction to things

Breast feeding is not always possible, some women are unable to nurse for various reasons and in some rare cases babies can have an allergic reaction to things in their mother's breast milk (that makes it way in from the mother's diet). There are probably other reasons I'm not aware of that could be in play, too.