Paley has said, "Every species practices being young." Humans practice the same way other species do, with the added dimension of imaginative play. Children practice pretending to be someone else, somewhere else. They step outside of themselves and invent abstract thought. Children are inventing learning as they play with ideas.
When our at-risk students come to Head Start, we don't have the luxury of time. In many circumstances, we are trying to pack three years of play into nine months of school. You can't do that and just play in a classroom. So we try to find balance and "teach" some basic readiness skills like recognizing letters, numbers, colors, shapes, and make our lessons as "fun" as possible. But there is a difference between play and fun. Play occurs in children naturally when they are ready, teaching happens when the teacher is ready, and it will never, ever be as powerful as a child "inventing learning" for themselves.