To answer "Cave Man Mind"'s "why? and add to Rhi's awesome answer, some modern parents believe in the Nine-In-Nine-Out idea, where baby is in the tummy for 9 months and then in a sling/wrap/etc for another 9. Another term for this is the "Fourth Trimester."
Personally, I use slings because I'm too lazy to drag a stroller in and out of the car and I think people dragging bucket carseats around, bouncing off their knees because they are so heavy look ridiculous!
Man! I love Rhi's answer!
Some mammals, like bunnies for example, only nurse their young twice/day. Human newborns need to nurse at least every 2 hours in the beginning at least, so they really can't be plopped in a nest and left while CaveMommy goes hunting for berries. If CaveMommy goes too long without nursing her breasts will get sore and maybe even lead to infection so it's easiest to just keep baby with her to facilitate easy breastfeeding.
Experts say human babies are born with just three basic reflexes: sucking, swallowing and breathing -- and even breathing can be irregular. This may have to do with the immaturity of the human newborn's brain, which is only about 25 percent of its adult weight at birth, while most other mammals are born having 60 percent to 90 percent of their adult brain size.
Some of the answers are found in wikipedia's babywearing answers:
Benefits of babywearing include:
Mothers' oxytocin is increased through physical contact with the infant, leading to a more intimate maternal bond, easier breastfeeding and better care, thus lowering the incidence of postpartum depression and psychosomatic illness in the mother.
Infants who are carried are calmer because all of their primal/survival needs are met. The caregiver can be seen, heard, smelled, touched, tasted, provide feeding and the motion necessary for continuing neural development, gastrointestinal and respiratory health and to establish balance (inner ear development) and muscle tone is constant.
Infants are more organized. Parental rhythms (walking, heartbeat, etc.) have balancing and soothing effects on infants.
Infants are "humanized" earlier by developing socially. Babies are closer to people and can study facial expressions, learn languages faster and be familiar with body language.
Independence is established earlier.
Attachment between child and caregiver is more secure.