In "To Build a Fire," what is the purpose of the Naturalist view of human beings?
Naturalism states that the role and effect of Humans in Nature is dependent more on Nature's effect on Humans, not the other way around. In this sense, it is somewhat deterministic, but more in the sense that Humans cannot hope to understand and master Nature, and those who are arrogant are doomed to failure. In "To Build a Fire," the protagonist is explicitly stated to have no respect for the intense cold of the Yukon, and despite his understanding of the dangers he makes mistakes that lead to his death.
[The cold] did not lead him to meditate upon his frailty as a creature of temperature, and upon man's frailty in general...
Try as he would, he could not clutch hold of it. And all the time, in his consciousness, was the knowledge that each instant his feet were freezing.
(London, "To Build a Fire," jacklondons.net)
His view of the world is "without imagination," and so despite his intellectual knowledge of the dangers of the cold he is mentally unprepared for the realities of it. The Naturalist view shows this explicitly, since he is not concerned with Nature as something to be experienced but something to be conquered. Instead, he as a single human is unable to force his will on Nature, and instead Nature crushes him without intent or malice. The Naturalist style of the story shows how there is no romance in freezing to death, only the death itself.