Health benefits  

Acts as a back support for the wearer (similar to weight belt/motor cyclists belt/lumbar support) and as a very effective way of redistributing the weight from the neck, spine and shoulders.  The more tightly the belt is worn the firmer the support on both counts.  The weight is spread around the waist, hips and pelvis and the support comes from under the child rather than weight being borne from above with consequent bending, stretching and strain. 

Health professionals including osteopaths, physiotherapists, & chiropractors have reservations about forms of carrier that require weight to be borne by over the shoulder using slings or harnesses.  These are very good for very young children but after about 6 months (depending on the weight of the child) they can actively strain the spine and muscles, adversely affecting one's posture and pulling the back out of shape.