Mary Shelley writes with a frame for several reasons. First, the use of letters was a common practice of her era. This also began the story with a narrator (the author to the letter) who had no bias from Victor's point of view. The narrator simply tells the story from his eyes looking in on the poor excuse for a man Victor had become. This also foreshadows the end. Readers get a taste for the finished product before knowing the entire history of Victor's journey with his monster.
This frame also serves to help develop the doppelganger effect. This effect which essentially means that a character's shade or shadow follows them is an interesting feature of this text. In the beginning (which is really the end), Victor is chasing his monster. It is ironic that Shelley positions the main character this way. It is almost as if to suggest that often man chases his own demons instead of being haunted by them.