What is the role of the narrator in "A Sound of Thunder"?

The third-person narrator of "A Sound of Thunder" sets the scene of the various parts of the story as seen through the eyes of Eckels, who is the main character. The narrator also puts the reader inside the mind of Eckels so that we know what his thoughts and emotions are as the story unfolds.
The narrator conveys both what Eckels physically experiences and his inner sense of wonder. For example, in the paragraph below, the first sentence show us the sight and sound of the time machine as Eckels experiences it. The second, much more lyrical sentence, tells us how the time machine seems imaginatively to Eckels:

Eckels glanced across the vast office at a mass and tangle, a snaking and humming of wires and steel boxes, at an aurora that flickered now orange, now silver, now blue. There was a sound like a gigantic bonfire burning all of Time, all the years and all the parchment calendars, all the hours piled high and set aflame.

For much of the central part of the story, the narrator disappears and we overhear the dialogue between the characters. But it is the narrator who introduces us to the T-Rex, describing all the wonder and terror of Eckels seeing one for the first time:

Each lower leg was a piston, a thousand pounds of white bone, sunk in thick ropes of muscle, sheathed over in a gleam of pebbled skin like the mail of a terrible warrior.

The narrator also recounts Eckels leaving the trail, the killing of the T-Rex, and the reactions of all the men. At the end of the story, the narrator allows us back into the mind of Eckels, as he returns to the present day. We experience the eerie sense that Eckels does of everything being not quite right:

Beyond this room, beyond this wall, beyond this man who was not quite the same man seated at this desk that was not quite the same desk ...

The narrator is able to get beyond dialogue to describe what is going on emotionally with Eckels. And finally, the narrator has the ability to stand back and describe the sound of thunder as Travis shoots Eckels.