"The importance of play on a young child's development cannot be understated. Play gives children opportunities to understand the world, interact with others, express and control emotions, develop symbolic capabilities, attempt novel or challenging tasks, solve problems, and practice skills. Play may contribute to the development of posture, movement, and self-sufficiency. There also appears to be a positive relationship between the frequency and complexity of young children's play and IQ, problem-solving, creativity, language and literacy, and social competence."
Developing a Preschool Play-based Curriculum by Mary Frances Hanline, International Journal of Disability, Development and Education, Volume 46, Issue 3, September 1999, pages 289-305.