Victor's tale, from Mary Shelley's Frankenstein, is very important given the voyage Walton is on and the mistakes Victor fears he will make (regarding his obsessive quest for knowledge).
In Letter I, Walton reveals his obsessive intent of traveling to the seat of magnetism (the North Pole).
Six years have passed since I resolved on my present undertaking. I can, even now, remember the hour from which I dedicated myself to this great enterprise.
One man's life or death were but a small price to pay for the acquirement of the knowledge which I sought; for the dominion I should acquire and transmit over the elemental foes of our race.
Victor, knowing that Walton will not stop until he reaches the pole or dies, fears for his friend. He knows the consequences of pursuing knowledge. He also knows the turmoil one faces when in search for knowledge. 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.