This frame style approach actually contributes greatly to theme. If you think about it, Victor's character was very concerned about the ability to create life.
If there is a creator God, he created mankind. Mankind in turn gets to create additional mankind. Each generation has a place in history in which they live, but the part they play is a small one in an even bigger storyline.
The same is true for the storyline of Frankenstein. The perspective of Robert Walton and his letters to his sister may represent that creator God storyline. This would be all of human history. Then, a man that has been created by God, whether it be Adam of the bible or Victor Frankenstein of the novel, serves as the next frame. Finally, the inner frame is the story of the last created. This could either be a current generation or the creature in the story.
This helps not only explore the theme of creation, but it also leads to the issue of being responsible for the life created. In society, we see this with each new generation's spin on parenting their children. In Frankenstein, we see Victor's weaknesses and conscience as a parent figure of the creature.
Because of the frame structure, readers are encouraged to think about how their own life stories interplay with stories much larger than their own.