Why do children need unstructured time outside?

[Artsand crafts]

I found this documentary (the trailer) in another forum. It's called "Mother nature's child".

"Mother Nature’s Child explores nature’s powerful role in children’s health and development through the experience of toddlers, children in middle childhood and adolescents. The film marks a moment in time when a living generation can still recall childhoods of free play outdoors; this will not be true for most children growing up today. The effects of “nature deficit disorder” are now being noted across the country in epidemics of child obesity, attention disorders, and depression"