Play is an interesting thing. Sometimes it is loose and unstructured, changing to suit the players and the circumstance; sometimes it is codified with rules, rewards, and consequences to suit the game. Even with our dogs – agility rules vary by organization and the structure of personal play changes depending on the dog and person!
Play is studied more often than most people realize and has been studied from a number of different directions – one essential work is The Genesis Of Play (2005) authored by Gordon Burghardt who carefully explored the ways play has been considered in regards to development and behavior across species. While he ultimately posits that it is not essential, nor always beneficial, he contributed some fascinating insights into play.