I received one Christmas, from my mother, the box sets that contained the rules for Dungeons and Dragons, and I was quite intrigued by the esoteric nature of the concepts within. As compelling as the ideas were, the actual process of playing the game was a confusing jumble of jargon and text scattered throughout several booklets. My mother was even concerned that it was a bad gift, but I assured her it was ok, that I liked it quite a lot. Even once I got the gist of it, however, I had no way to play the game as it required a group of friends that I considerably lacked at that time.
When I went to college at the Savannah College of Art and Design, I lucked into getting invited to play with a group of students that saw a castle and dragon doodle I did in class one day. From there we managed to play a version of the game that kind of fit our playing style. I don't think we ever knew much beyond the core mechanics.
Those core ideas made up the world that I would go on to create and run with another group, and they became the core ideas that made up the world I would eventually write about. It does sound silly, but when you wind up creating original history, myth, politics, social orders, gods and religions, a multiverse, magick, nations, towns and villages across an original 'world', you tend to want to use it for other creative projects.