There are many reasons why breast milk is superior to the use of formula for infant nutrition.
Breast milk includes antibodies the mother has developed over the years of her life. Passing these on to the infant provides initial protection against various types of infectious illnesses.
Babies who have been breast fed show significantly lower rates of illnesses such as middle ear infections, diarrhea, upper respiratory tract infections, and urinary tract infections. Breastfed babies seem much less susceptible to SIDS (Sudden Infant Death Syndrome).
Breastfeeding seems to reduce the chance of the child later developing diabetes, obesity and may help to reduce the risk of developing allergies.
For all of these reasons, breast milk is a better option whenever possible.