What is the effect of the last stanza mimicking the first?

Excerpt from The Raven (excerpt)

D) The tapping, which was seemingly innocent, is now seen as mysterious by the speaker.

In the first stanza, the speaker heard the tapping which woke him from his nap and muttered about how there must be a visitor. It's clear though, from the last stanza that there is some sort of mystery about because when he went to the door there was no one there.