It's convenient. You can breastfeed discretely in a sling while working, getting things done around the house, shopping or eating in a restaurant. If you have a baby who wants to be held all the time, this is an easy way to meet her needs while doing other things.
It helps moms care for older siblings. Carrying your baby in a sling can give you the mobility to care for your older children.
It helps some babies to breastfeed better. Some babies, particularly those who are tense or tend to arch their backs, breastfeed better while moving. Also, babies who are slow to gain weight (for no apparent reason) have been known to gain better if carried in a sling for several hours a day, since proximity to mom encourages babies to eat more frequently.
It reduces crying and colic. A 1986 study of 99 mother-infant pairs (reported in Pediatrics) showed that carrying babies at least three hours a day reduces crying and fussing 43% during the day and 51% at night. Babies are happier because they have less need to cry, and parents enjoy their babies more as a result.
It enhances learning. Carried babies have enhanced visual and auditory alertness, and increased "quiet alertness" times. Carrying a baby promotes cognitive development and speech development, since babies are exposed to more experiences and conversations.
Carried babies are involved in their parents' world. They participate in life, rather than see it as a spectator.
Carrying your baby promotes bonding and enhances parents' feelings of competence. A higher frequency of feeding and touching stimulates mothering hormones, and frequent carrying encourages and speeds the development of a mutual reading of each other's cues.