Although Robert Walton's opening and closing letters, in Mary Shelley's Frankenstein, frame the story, they do much more. Given that expositions to stories offer the introductory information needed to set up the path the text follows, Walton's letters are no different.
The letters introduce an idea that comes to be the reason behind Victor's downfall: ambition. In Walton's letters, the reader comes to understand Walton's passion and his refusal to give up on his dreams--to discover a shorter path for traders over the seas and to discover the seat of magnetism in the pole. Many times in the letters, Walton openly states that he is not willing to give up. He even states that his expedition may keep him from being able to come home (given it could be his death).
As readers get further into the novel, they can come to understand that Walton and Victor are very much alike. Both are obsessed with success. These obsessions eventually lead to the death of one, Victor. While Walton's future is unknown, Victor leaves him with such a haunting tale that he (Walton) begins to question his own plans.
Therefore, Walton's letters most certainly do more than simply frame the story. Instead, they offer readers a chance to make connections between two people with similar ideologies and goals. Also, the letters set up an introduction to the theme of Forbidden Knowledge and the high cost of seeking it.