When kids play, structure is often not what they are consciously thinking about, unless they are rule sticklers.  Sometimes, adults want to give them structured guidelines to their play, thinking that it will improve it.  Although some sense of structure is good sometimes, purely creative play is also a good thing.  Most purely creative play consists of just running around pretending or making up games, not changing rules of already structured games. 

Structured play is not a problem.  It teaches a child to follow rules, since the rules are the structure.  It is good for kids to learn that often times they have follow rules in life and it is not complete freedom and anarchy.  Structure also teaches fair play. If he has to abide by the structure to play the game, he cannot cheat. Structured play also includes induces creativity within the confines.  Working always within those confines though can also though create a comfort zone, in which he learns to rely on the structure.

Play that is not structured is when a kid runs about pretending he, himself, is a hero, on a safari or something equivalent.  Alternatively, he might be participating in role playing, not the structured type with rules, but when he pretends to be someone else.  Often, this sort of play involves transforming his surroundings into his character’s imaginary surroundings.  A play structure might be a pirate ship, a castle or a space ship.  An open field or backyard can be transformed into a savanna, a battle field or a jungle.  

This sort of play, without an imposed structure, gives the kid freedom to dream, learn how to form his own structure and the ability to be creative without a structure, in addition to within one.  Kids need the freedom to dream and run wild in their play.  It is one of the hallmarks of childhood.  Play is part of how they build their personality, self-identify and form bonds with other kids, on their adventures.  Play is also how they explore the adult world. This why little girls play house.  Also, play how they learn to take a myriad of chaotic ideas or nothing and creates their own structure that makes some semblance of sense, even if it is childish nonsense.  Not everything in life has an already imposed structure. 

Play and imposed structure should not always go together.  Although playing within a structure is also a good thing, the freedom to create their own structure is a good thing to develop. Besides, it makes a play structure at the park more fun.