in the 42 years since, he has interviewed some 6,000 people about their childhoods, and his data suggest that a lack of opportunities for unstructured, imaginative play can keep children from growing into happy, well-adjusted adults. “Free play,” as scientists call it, is critical for becoming socially adept, coping with stress and building cognitive skills such as problem solving. Research into animal behavior confirms play’s benefits and establishes its evolutionary importance: ultimately, play may provide animals (including humans) with skills that will help them survive and reproduce.
Source: here