An animal behaviorist offers to explain:
People tend to think of play as an activity one engages in at one’s leisure, outside of learning important skills needed to succeed later in life, such as hunting, mating, and evading predators. But although playing is fun for all involved—and fun for those who are watching—play behaviors evolved as ritualized forms of survival skills needed later in life, providing the opportunity to perfect those skills.
Engaging in play allows animals to experiment with new behaviors in a protected environment without dangerous consequences. The unwritten code of conduct surrounding play lets them explore many possible outcomes.
Animals learn the rules of engagement for play at a very young age. Among dogs, the bow is a universal invitation to engage in silliness that triggers the same bowing down and splaying of the front legs in the receiver of the signal—inevitably followed by chasing and pretend biting. Chimpanzees and gorillas motivate others to romp by showing their upper and lower teeth in what primatologists refer to as a play face, which is comparable to human laughter.Caitlin O’Connell, “Play Is Serious Business for Elephants” at Scientific American
Okay. Anyone who grew up with cats will recognize this: Almost from the first time kittens can stagger around, they engage in play fights. Kittens find it hard to stop until they collapse from exhaustion.
One thing play probably does is enable the kitten to develop its neurology. The neurology is a potential until the connections are made.
See also: In what ways are cats intelligent? Cats have nearly twice as many neurons as dogs and a bigger and more complex cerebral cortex.