To be honest, it is almost always the fluff that makes me buy the game. If the premise is interesting, then I am hooked. If it is "Zombie-Survival-Horror-Shooter #459,001,632" then count me uninterested. There are exceptions of course, the old Rise of Nations games just let me rule the freakin' world, let's me satisfy a minor God-complex of mine.
But anyways, fluff makes a game for me, and what was often great about the manuals (in-game or PDF really) was the behind-the-scenes extra fluff inside said text. Homeworld, Warcraft, Starcraft, Legend of Zelda, etc. were brought that much more to life by the accompanying text.
Even Halo continues to bring me back simply because the tale of the Covenant's demise is a good one, one that too often is ignored. The Covenant themselves called it the "Age of Doubt". And it was, it was the fall of a once great, just, and noble alliance, one that fell to politics, greed, and the revolution of a warrior-elect that saw the honour of their cause fall away.
For some of us, fluff is everything.