In Frankenstein, why is Victor's tale important to Walton?
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.
Victor's tale is important because it serves as a warning to Walton. He eventually turns around and heads back through the ice, as his crew requested. Victor and Walton are both very similar characters (they could be argued to be doubles/ doppelgangers) and both express Narcissistic characteristics. It is therefore important that Walton hears the story of Victor as they both are over-reachers through their desire for exploration and knowledge. Hearing Victor's story may also serve to realign Walton's moral compass.