The Elements Support a Theme
The author asserts that a game can have a theme, similar to the theme of a novel or a movie, and that a compelling theme can contribute to the experience of the game. Theme also blurs the line between a "play" experience and a work of artistic expression - though the latter should not overtake the former.
The theme also fits into the psychology of gaming as an escape - the player becomes immersed in the theme of the game, and that immersion adds to the entertainment value.
Theme is also part of the marketing of a game: a game fo a certain theme has a certain appeal to a certain audience. Some players are fans of a specific theme (military, horror, etc.) and gravitate toward games that are cast in their theme of preference.
And often, when a game is "controversial," it is the theme of the game that causes the controversy.
The theme can drive the story, can have an impact on the mechanics, and very often has a strong influence on the aesthetics.