Breastmilk provides calories, good fats, and antibodies that protect their microbiomes from opportunistic infections and illnesses.
Breastfeeding reduces the feeding parent's risk of breast and ovarian cancers, diabetes, postpartum depression, and may delay the return to fertility.
The World Health Organization recommends babies initiate breastfeeding within the first hour of life, receive breastmilk exclusively (when possible) for the first 6 months, and continue to get some breastmilk for at least the first 2 years of life.
Victora, C., et al (2016). "Breastfeeding in the 21st Century: Epidemiology, Mechanisms and Lifelong Effect." The Lancet 387 (10017):475-490.