A delicate balance between freeform writing/creative storytelling and characters' abilities to interact with said environment and other characters.
Imagination is the only limit. Players will always be the primary drivers of the story. However, quantification is necessary to ensure reasonable possibilities which in turn allow reasonable moderation of the success or failure of theses players' actions. The game's systems exist to try to put rough boundaries/schematics on this blank slate of an imaginary world, in which players can use things like write-ups to narrow down theme, scope, and/or what their character should be expected to do, and then they can freeform write within that framework. The moderator then reconciles the two things to develop a possible outcome.
The system is also inherently cumulative. The longer you play, the more you have at your disposal to help "move the story" along, whether this be acquired items, experience, abilities, and/or relationships. This isn't to say newer players don't have anything, but the game trends toward acquisition, so the longer one plays the more they are adding things, abilities, skills, opportunities, etc., to the character rather than taking them away.