Later, Lovecraft would use the Necronomicon less as a framing device for monsters and more as a hint of the antagonists' true nature. Merely owning the book is enough to show that the owner is up to something seriously dodgy, and wanting to read it, as Wilbur Whateley did in The Dunwich Horror, is proof of guilt; a literary equivalent of the smoking gun. In each instance Lovecraft used the Necronomicon as a means of directing the narrative, showing the reader what was really going on and hinting as to what might happen next.
Call and Trail of Cthulhu are investigative games. The characters, often with no prior knowledge or interest in the occult, are thrust into a situation that they did not wish for and then must solve the mystery. If they fail, the forces of the Mythos win, and since this usually means destruction of possibly of the entire world, stopping the forces of the Mythos soon becomes the protagonists' only mission in life. Sandy Petersen, one of the authors of Call of Cthulhu, likened the process to peeling an onion. "On the surface, suppose that the scenario looks like it's about a conventional haunted house. It might even look like a hoax. As the investigators penetrate the first layer, they should discover another beneath ... " Each layer in this instance being a further truth of the Mythos revealed, driving the narrative forward as the protagonists solve the mystery.
In this narrative, the grimoires, among which the Necronomicon is the most sought-after, have several roles. They're a source of general information and spells that boosts the character's knowledge base, allowing them to at least make an informed guess as to the opposition's strengths and weaknesses, as well as giving them a means by which they can retaliate. This gives the players a certain amount of control over their own destiny, while at the same time reinforcing the very real nature of the threat they face.
Of course, there's a risk. The grimoires contain occult knowledge that the characters need, but the price of that knowledge is a steady drain on Sanity, the mental statistic that defines the character's psychological wellbeing. Lose enough of that and the character becomes permanently insane. Each grimoire carries its own Sanity penalty; the more knowledge gained, the higher the cost, with the Necronomicon being the most important and therefore the most dangerous. Again, this offers control over the narrative, but this time expressed as a negative consequence. Do the characters risk their souls to save the world, or would they rather take their chances and hope that lack of information doesn't destroy them all?
Thus the narrative becomes a kind of balancing act: the need to preserve the self by keeping Sanity at a high level, versus the need to know more at the expense of Sanity. All this is fuelled by the grimoires, the ultimate source of occult knowledge. It's a kind of magic in its own right - the ability to change the ongoing story permanently, by tempting the player with the ultimate sacrifice.