There's several reasons that aren't necessarily requirements. One of things that makes a story and a game great, is atmosphere, world-view/world-setting, and it's ability to make you think.
A good atmosphere is what allows for better reader-immersion. When every element--text, sound, visuals--all come together to generate a solid feeling or emotion, readers feel the most immersed in the tale, and fully into the game.
A thorough world-view or world-setting is the backbone for that atmosphere. A setting close to home (Japanese high-school for the target audience of most games) is a short-cut that works by evoking all the knowns from a reader's own experience, but then the challenge is making the unique elements stand out and gain prominence. To create a (good) original or unique world setting though, you have to put in a fair amount of research and work into establishing it--often on elements readers may never see or hear. Like, if you're fighting armies, what's their tech and training level? What are their beliefs and why do they fight? A good backbone will have a lot of detail on that, even if readers only ever glimpse the surface result.
Any good tale that makes you think has conflict, and often conflict comes from either danger or clashing ideologies. Danger is simple to grasp, but even so, methods of responding to it and solving it will vary as widely as there are ethic viewpoints, and honestly, knowledge of philosophy is good background for developing those viewpoints and ideologies--especially believable ones. Still, these are things that inform the writing, and not necessarily obligated for readers to know--but knowing adds to understanding and increases depth of enjoyment. So I think that's where a lot of "you must read Kant" comes from--people recommending the title want others to have the full experience.
That all being said though, I absolutely have had people say "You must read X before Y"--in fact, when I was in school, that was part of our required reading for English! In my English clashes we were required to read up on the Bible and Greek Mythology, with mini-reports and assignments due on each prior to moving onto works and literature that drew on both of those. Reading Milton sure wouldn't make much sense without a background in Christanity and Greek Classics!
But that's because all works of literature are inevitably based around a general assumption of shared cultural knowledge--Shakespeare and Milton wrote their works knowing and assuming that most educated people in European cultures had studied Greek Classics and philosophies at least in passing, and that everyone grew up with Christian teachings from the church. This whole idea of drawing on shared cultural knowledge frankly goes back to that idea of creating your world-setting backbone: by drawing on common knowledge, you can take shortcuts, showing your reader actions and behavior befitting those philosophies instead of explaining new ideologies.
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