Gross motor skills are required for children to be able to control their large muscle movements for sitting, crawling, walking, running and climbing. Muscle control and co-ordination develops from the top down (starting from the head) and the inside out (building out from the torso to the fingers and toes). This ensures that the large muscles needed for moving and co-ordination are strong and in control before the child develops their fine motor skills (required for writing, and small precise movements).
So it is really important for children to build up their upper body by swinging off bars, climbing, hanging, and any other high-energy activities. For a more detailed explanation of this go to the Moving Smart Blog, ""M" is for Monkeybars".