Walter serves many purposes in the novel, all directly or indirectly related to a theme. Some of these purposes include offering parallels to both Victor and the creature, reflecting the tension between rationalism & romanticism, and providinga filter through which Victor's biases & perspectives are revealed/concealed.
Through Robert Walton, Victor's story is filtered by an unbiased third-person narrator. Walton is separated from the action, so he can remain impartial in areas where Victor cannot. He is also a man of science, a practical man. this leds an air of realism to an otherwise unbelievable story. Walton's letters begin and end the novel, creating a frame for Victor's and the creature's narratives. In this way, Walton combines the most important qualities found in both Victor and his creature. As an Arctic explorer, Walton, much like Victor, wishes to conquer the unknown. We find out that his crew is near mutiny due to the pressures he has put on them. Yet he does what Victor continually failed to do throughout the novel: he listens to the creature's anguished tale with compassion and empathy.
We also discover that Walton originally wanted to be a poet, but, having failed that endeavor, he embraced the scientific advancements of the age and became an Arctic explorer. Walton's letters begin and end the novel, framing Victor's and the creature's narratives in such a way that Walton embodies the most important qualities found in both Victor and his creature. Walton, in other words, balances the inquisitive yet presumptuously arrogant nature of Victor with the sympathetic, sensitive side of the creature.
In Mary Shelley's novel "Frankenstein" the letters sent home and narrated by Walton serve to give a different point of view from a neutral bystander. It is through him that the creature is able to express his feelings at the death of his creator/father, Victor. The majority of the story is presented through Victor's perspective, but Shelly wanted to take the reader into another person's perspective. Walton's letters and narratives also provide the reality of how far fetched Victor's story seems. However, Walton finds the truth in Victor's tale when the creature comes after Victor dies. His job is to preserve the story ad to complete it after Victor's death.