Poe starts out with a very measured pace and includes specific details on the setting, the ship and the narrator. That continues throughout the piece, even as the narrator becomes more unhinged and his tale grows more horrific. Only at the very end does he drop the observations and start screaming in outright horror. And even then, he makes sure we understand the circling of the ship around the whirlpool and "a roaring, and bellowing, and shrieking of ocean and of tempest" (27). We're pretty sure if we were circling the toilet bowl of the world, we wouldn't be flourishing our last words with metaphor. But hey, he's a stickler for detail.
That tone produces two specific effects. First, it conveys some of the narrator's personality (i.e., he's dull and non-opinionated). Second, it emphasizes the suspense by assuring us that the horrors of the hurricane and black galleon are actually real—as real as the original contents of the cargo hold or the cloud that announces the hurricane—instead of being the wild flights of fancy of a crazy person. It also heightens the mystery, since we can't be sure if the deliberate descriptions are accurate, or if the narrator has simply gone bananas. In either case—who knows what'll happen next?