Plato and Aristotle found that play was important in the education of children. Furthermore, Locke, Rousseau, Deway, Montessori and others reinforced that play should be an integral part to educational programs. The National Recreation Association in the 1900's developed guidelines for certain equipment for playgrounds like providing a sand box, swings, a small slide and a climber. This is known as the traditional playground. Unfortunately not much has changed since then.
Many people believe play is good for burning off energy and for physical growth.
Therefore, traditional playgrounds are fine because they satisfy this requirement. Most of the child's development is from unstructured activities which most do not comprehend. The developmental benefits of play include cognitive developmenf, increased imagination and creativity, increased discover and reasoning, manipulative skills and though development and improved problem solving. Less numerous, more costly, liked more by kids than traditional type, encourages educationally worthwhile forms of play. Bruya, L.D. Play Spaces for Children .