Question

What is Mary Shelley’s purpose for writing the introduction to Frankenstein?

Answer 1

Ignoring the fact that she needed a 'sole' purpose.
I believe Mary was expressing a fear that persists to today that scientists and doctors are playing 'god' with regard to nature. She expressed a slippery slope scenario where the doctor's creation slipped beyond his control.

Answer 2

To explain how she wrote a terrifying novel at such a young age