It is this fear which propels Victor to share his tale with Walton (compounded and defined with the use of "Rime of the Ancient Mariner"). Like the ancient mariner and the wedding guest, Victor feels it necessary to provide Walton with a warning of acting without thinking of the consequences.

Victor's tale worked. In Walton's reclaiming of the narrative voice in chapter twenty-four (remember, the entire novel is from Walton's voice given he is retelling Victor's tale for him), Walton admits his horror at Victor's tale.

You have read this strange and terrific story, Margaret; and do you not feel your blood congeal with horror like that which even now curdles mine?

In the end, Victor's tale did what it needed to do: changed Walton's mind about pursuing his quest.

I would not lead them farther north if they strenuously desired the contrary...The die is cast; I have consented to return if we are not destroyed.

The frame story serves to highlight the contrast between Frankenstein, the mad scientist, and normal, decent society, as represented by Robert Walton. Though Walton has much in common with Frankenstein, as we shall see, there's no way in a million years he would ever entertain the kind of deranged fantasies in which Victor has indulged. In that sense, Walton acts as a foil to Victor, and the use of a frame story is an especially effective means of bringing out the personal characteristics that set him apart from the crazed scientist.

The employment of a frame story also serves to make Victor's actions and their disastrous consequences seem more real, less fantastical than they otherwise would be were they to be related in a more conventional narrative structure.

The main connection between Victor and Walton is that they are both quite lonely souls, isolated from society by their singular personalities and their single-minded pursuit of scientific truth. That is why they instantly connect with each other, despite certain radical differences in character and temperament that might otherwise have kept them apart.

Although Walton himself would never have entertained the notion of creating some kind of monster, he can at least sympathize with Victor's goal of pushing back the frontiers of science. He too, in his own voyage to the frozen north, wishes to do precisely the same thing, albeit in a much more conventional manner.

Frankenstein has a complex narrative structure. There are actually three "frames"—the story of Walton and his expedition to the Arctic, the story of Frankenstein himself, and the story the monster tells Frankenstein of his life after his creation. Add to this that the entire story is told through a series of letters Walton writes to his sister, and that there are several times in the novel where Walton's letters (and Frankenstein's and the monster's stories) quote the words of other characters, and you get a sense for how layered Shelley's story really is. Generally speaking, early novels took on the epistolary form in an attempt to make them more "real"—the idea was, if the reader was presented with an incredibly thick packet of letters rather than a "novel" per se, the text would seem less like "fiction." So, odd as it may seem, one motive for Shelley's constructing the novel as she does is to make it more "realistic."

For modern readers, the frame is fascinating for another reason—it dramatically problematizes point of view. Since the story is told entirely through Walton's words, the narrative structure calls into question the "truth" of Frankenstein's story (Walton could have made it up). Even assuming (as I think we must) that Walton faithfully transcribes Frankenstein's story, as readers we never have any firsthand knowledge of Frankenstein, much less the monster. The way the novel is presented to us precludes any certainty about what really happened; this uncertainty about the production of the text mirrors the uncertainty Frankenstein feels about the production of his "text," the monster, and it mirrors the ultimate unavailability (to Frankenstein and to the reader) of the monster's experience. So, there is a sense in which the framing structure of the book serves to amplify one of Shelley's basic themes, which is the problem of "otherness," or of understanding the interiority of other people.

I think you could also consider how this makes the entire impossible story a bit more plausible.  Walton and Victor are both explorers, pushing the bounds of knowledge and science into places where it might best not go, and becoming isolated from those that love them.  Walton's letters, and interesting device in themselves, present something that we believe could be "real."    If we buy into the reality of Walton, it becomes a little easier to buy into the strange story of Victor.  And if we can buy into Victor's strange tale, when we arrive at the onion's core, the Creature's story, we might just be a little more likely to buy into it.  And then we exit the core through the Victor layer and back out to Walton level and "reality."  It's and interesting device and I think it works well!

In the opening letters of Mary Shelley's Frankenstein, Walton is explaining to his sister the importance of his voyage to the North Pole (in order to find the source of magnetism). Walton details that his "expedition has been the favourite dream of my early years." At many points throughout the letters, Walton wishes his sister blessings given he may not be home for many years, if he makes it home at all.

Here, Walton states his understanding that his expedition may be a dangerous one. Unfortunately for his sister, by offering this farewell, Walton is admitting his obsession with completing his search for the seat of magnetism. Walton will risk death to find it.

Much later, when the narrative returns to Walton's letters (after Victor's tale has been told), Victor tells Walton about his mistake.

“When younger,” said he, “I believed myself destined for some great enterprise. My feelings are profound; but I possessed a coolness of judgment that fitted me for illustrious achievements. But this thought, which supported me in the commencement of my career, now serves only to plunge me lower in the dust."

In this statement, Victor is telling Walton about the evil of ambition. Essentially, if Victor would not have been so ambitious, his life would have ended up very different.

In the end, Victor's tale offers a warning to Walton. Much like the many allusions made to "The Rime of the Ancient Mariner," Walton (the "wedding guest" ) is receiving his warning from Victor (the "ancient mariner"). One can easily assume that Walton, especially after witnessing the Creature himself, understands Victor's warning:

I am surrounded by mountains of ice which admit of no escape and threaten every moment to crush my vessel. If we are lost, my mad schemes are the cause.

There are several reasons that Mary Shelley provides the frame story in which Victor Frankenstein’s tale is set, and makes the character of Robert Walton the person with whom Victor shares it.

One reason is practical and structural. By creating an audience and a remote setting, Shelley gives Victor an audience for his story and an isolated location in which he might speak at length, undisturbed. The character of Victor also needs to confess what he has come to view as his transgressions, so the form of an extended personal narrative fits with this aspect of his personality development.

The setting is also more than a convenience. Victor has made an arduous physical journey to these far reaches of civilization. The physical journey parallels his spiritual journey, in which he has traveled comparable distances in in his mind and soul. Although the discomforts of the new environment are considerable, Victor would not have felt comfortable sharing his story with anyone in his social circle. He can only make these guilty admissions in a strange environment and to a stranger.

The features of Walton’s character are also carefully developed. Shelley has created a foil to Victor, a character who has also experienced considerable soul-searching but, unlike Victor, has managed to hold onto his basic humanity and humility. He can also be contrasted to the creature, who—as Victor’s alter ego—embodied more of his negative characteristics. Robert's letters to his sister are both part of his self-analysis and a way to fix the oral quality of Victor's story as written text.