Not everyone plays RPGs so that the GM can tell them a good story. Not everyone regards the game as belonging solely, or principally, to the GM. For some, story, backstory, campaign world etc is secondary to game play. For others, they want to contribute in a significant way to the telling of the story.
When the focus of the game is on game play, then there needs to be some way of mediating possible competing moves. When the focus of the game is on story contributions, then there need to be some way of mediating possible competing contributions. This is one thing that rules can do. For them to do this, they have to have some content that is independent of the GM's conception of them, given that for them to do this they have to bind the GM as well as the players.
Both Hussar's examples (plate armour, and manticores) concern background elements. I think the scope for conflict comes up most often in relation to action resolution. For instance, can a powerful epic-tier fighter leap 50' from a flying carpet onto a flying slaad lord? This question came up in my most recent 4e session. We didn't answer it by considering what makes sense to me (the GM) or to the player of the fighter. We resolved it using the jumping rules.