Interestingly, after Frankenstein is satisfied by Walton's response that he is "on a voyage of discovery towards the northern pole," he talks with Walton about the expedition, expressing curiosity and sympathies about this venture:

He entered attentively into all my arguments in favour of my eventual success, and into every minute detail of the measure I had taken to secure it.  I was easily led by the sympathy which he evinced to use the language of my heart; ...and to say, with all the fervour that warmed me, how gladly I would sacrifice my fortune, my existence, my every hope, to the gurtherance of my enterprise.

However, as Walton continues, saying,

One man's life or death were but a small price to pay for the acquirement of the knowledge which I sought

"a dark gloom" comes over Victor's face.  Here, clearly, is an example of foreshadowing as the reader later learns.  Of course, the irony, too, is that Walton does not realize how much his words sound like those of the idealistic Victor who would create his own being.  To Walton, Victor speaks in "broken accents":

Unhappy man!  Do you share my madness!  Have you drank [sic] also of the intoxicating draught?  Hear me--let me reveal my tale, and you will dash the cup from your lips!"

This fourth letter also furthers the completion of the frame around Victor's history as it gives reason to the telling.

The answer to this can be found in the fourth letter of Robert Walton -- the last one before Victor Frankenstein's narrative begins.

In this letter, Walton tells of finding this man out there on the ice, near death, with only one dog left to pull him.  Walton tells of being astonished when the man will not get on the ship until they tell him where the ship is headed.

What Walton tells him is that they are on a voyage of exploration to the "northern pole."