As they master their world, play helps children develop new competencies that lead to enhanced confidence and the resiliency they will need to face future challenges. Undirected play allows children to learn how to work in groups, to share, to negotiate, to resolve conflicts, and to learn self-advocacy skills.
Directly quoted from the abstract of "The Importance of Play in Promoting Healthy Child Development and Maintaining Strong Parent-Child Bonds" by Kenneth R. Ginsburg and the Committee on Communications, and the Committee on Psychosocial Aspects of Child and Family Health