First of all, because it is CONVIENIENT, for you and your baby- you can go whenever, wherever, and do whatever without feeling overwhelmed by the many things you have to carry with you ( such as car seats, strollers etc.).
Ever tried to squeeze into mall store isles pushing a stroller ? Well, then you know what I am talking about.
Speaking of convenience if you are BREASTFEEDING it is definitely a NECESITY, because you can breastfeed hands free, you wont need a breastfeeding cover and there is no better way to breastfeed in a public place without the concern that somebody would notice, because there is no more DISCREET way of breastfeeding than with a sling! Also after the 4th month the baby gets easily distracted when she eats, you can totally forget about that when you cover yourself with the tail of the sling.
Third, it is very BENEFICIAL FOR THE PHYSICAL DEVELOPMENT OF THE BABY- the sling is just like the mother's womb and puts the baby in the best position that follows the natural contour of his spine, as there is no direct pressure to the spine as in the baby car seats for example. Also the physical feeling is almost the same as in the womb. Your movement, your heart beat, your warmth, everything is the same and there is also a bonus to that - your smile when the baby looks up. The difference between the kangaroo and the sling is that the sling does not pressure the baby's pelvis while it is still soft and forming, by pulling the legs apart. Don't believe me? Try staying in a parachute harness for an hour and tell me how your legs feel! Also the natural position of the baby before she is born is the oval form of the uterus. Then we start to “stretch” the baby and when you put it in a kangaroo you are really forcing the baby in the straight position which is very uncomfortable for her. The sling gives the baby the opportunity to adjust her position bit by bit. The babies carried in a sling develop a balance more quickly when they start to walk (as a bonus to that you can use your sling to help your toddler walk by putting it around his chest like a leash). Should we also mention the colic relief?The benefit for gastro-intestinal reflux?Well, I think you get the idea :-).
I have also to mention that there are PHYSICAL BENEFITS FOR THE PARENT TOO. Another difference between the carriers using straps ( as the kangaroo) and slings – the sling does not restrict circulation in the mothers chest tissue and thus it does not affect breastfeeding.