Moll Flanders is focused both on telling a good story and on making that good story seem true. We're supposed to take Moll at her word and believe wholeheartedly in all of her adventures, no matter how outlandish. Plus, the author of the Preface takes pains to remind us how true the story is (despite a few tweaks here and there). But hold your horses, folks. This is a novel. It's completely made up. Moll is not real, and nor are her adventures. Even the author of the preface isn't real, for how could he have met Moll and written her story? All these issues of truth and authorship beg the question: why does this novel go so out of its way to convince us of its truth? Why not just trust us readers to enjoy the ride?
The whole reason the story is told in the first person is so we don't worry too much about Moll's life being in danger. We know she'll live in the end, because she's alive to tell her tales.
The fact that this book is really written by a man gives the voice of Moll Flanders an authority that female authors lacked at the time. If Defoe felt this tale was worth telling, well then surely it's worth reading.