By the end of the poem, it's clear that "The Highwayman" is a ghost story, and we think the speaker of this poem tells it like that. We imagine this speaker being like a camp counselor, with all the campers gathered around the fire. He sets the tone (probably whispering) with all that stuff about darkness and galleons and moonlight. There are no big scares in this poem, but the whole thing has a spooky vibe, helped along by that moonlight that shows up everywhere.
Like any good storyteller, our speaker also throws in some sound effects, imitating the sound of the horse's hooves in lines 67 and 68: "Tlot-tlot; tlot-tlot." That's where we really hear the speaker's voice. He's not just reciting facts; he's trying to pull us into the story, trying to make his audience feel exactly how mysterious and intense everything is. When he gets to the end, he's done such a good job that the campers are hanging on every word.