What purpose does the repeated tapping sound at the door serve in Edgar Allan Poe's The Raven

The setting of Edgar Allan Poe's "The Raven" explains why the speaker of the poem needs "to reassure himself by repeating that the tapping is only some late-night visitor." First of all, stanza one tells us it is midnight, and a "dreary" one, at that. The speaker is sitting in his library or study, and he is "weak and weary" from studying or reading some of his old books (or thinking about them as implied by the word "pondering"). Even worse, he has nearly fallen asleep, but not quite, when he hears a tapping on his door--and we all know what it is like to be startled awake in a hazy pre-nap daze. Finally, the knocking is on his chamber door, the door to this room, which is presumably not the door to his house.