At its most functional a game is a world of actions, rules and strategies. You play, your play brain finds it fascinating and you enjoy the experience of winning. However there is also your other cognitive side, your art brain. It looks for experience, inspiration, and the opportunity to create. When a game gets both sides right then something weird happens.
You notice numina. You infer qualities of the game that are not actually there. You care about it and catch yourself actively thinking beyond the game, to the point that the world seems to have an existence all on its own and you have just been lucky enough to step into it for a time.
It becomes a belief engine.