"Play" is the psychological/biological term for how children (of all species) learn important life skills by naturally finding them fun. Now that important life skills aren't obvious/instinctive because society is so complex, making those complex skills fun is the way to integrate them into the child's natural humanity. We don't need to make children miserable just for misery's sake out of some Puritanical ethic.
Nobody is saying to overload a child until they are unhappy. But the lesson needs to be learned that even if they don't naturally find it fun it still has to be done.
