Why do you think Shelley chose to include the letters at the beginning of the text? What function do they serve?

I am assuming that the question is in reference to Mary Shelley's Frankenstein.

Frankenstein opens with Robert Walton's letter from St. Petersburgh, Russia, to his sister in England for several reasons, both literary and psychological. There are many examples of works in English and European literaturethat begin with a tale within a tale: The Rime of the Ancient Mariner and Wuthering Heights readily come to mind. But whereas those are tale-within-a-tale narratives, Frankenstein is actually a letter written by a brother to his sister.

In order to understand why Shelly chose to situate the letter at the beginning, let me refer you to a very interesting book by the late Edward Said called Beginnings (1985). In this book, Said analyzes the beginnings of several types of literary and journalitic works and comes up with the hypothesis that, in the begiining of a work, a microcosm of the whole work is embedded. This is why, writes Said, perhaps with tongue in cheek, authors write the introduction to their works last!

Applying Said's theory to the letter with which Frankenstein opens, we find that this letter contains, not only the moral of the story of Frankenstein, but, in fact, parts of the letter signifies and even foreshadows what is to follow in the story proper.

To begin with there is that thirst for adventure and knowledge; an indomitable desire to know everything there is to know, to create that ultimate phenomenon that will answer all questions humans are capable of asking.

But beware: desire for knowledge caused the angels to fall!

The witnessing of the man-made monster is of course, quite literally a foreshadowing of the story, a near cinematic image before the days of the cinema! But it is more than that.

To Christian readers, this part of Walton's letter can be understood as the ultimate perversion of human desire -- the creation of a deformed creature because only God makes human beings in His own image. Not only, is the monster ugly, he is dangerous -- for he has a human brain to go with his super human strength.

Finally, the story. The letter ends with the announcement of the story, in the conventional tale-within-a-tale that sets up the actual story of Frankenstein as a prologue would a play.

I hope this answers your question.