Part of the manner in which the third person omniscient style helps understand much of Brian is through the telling of both a physical and emotional frame of reference. The narration style helps allow us to understand the harrowing conditions that Brian has to face and endure. Yet, it also probes into the emotional depth of Brian and where he is at all points in his experience. We understand his detached nature at the start of the novel and how it elevates into fear when the pilot suffers the heart attack. The third person style allows us to understand how frustrated Brian felt at the tornado ruining his home, but also how difficult "The Secret" was for him to carry. In being able to relay both the physical experience of his predicament and the emotions that accompany it, the reader develops a more whole vision of what it means to be Brian Robeson.
Having the novel Hatchet told through a 3rd person perspective, is really what sets the tone for this book. Since Brian is essentially the only character throughout the whole novel (other than the pilot who dies in the beginning), readers are constanly focused around him, his actions, and his thoughts. Having this third person power to be able and witness what Brian is going through physically and mentally, is what makes this novel so compelling and exciting to read. Being the only character, readers are able to emphasize and connect with him much more easily than if there were multiple people vouching for survival. Readers are able to see his transformation, some of his flashbacks, his thought process on surviving, and his deepest moments in the wilderness. Having the book set through this omniscient view that you mentioned, is really what makes the novel what it is.