The figure's speech is important to the story as a whole because it exposes the weakness of public morality at the same time that it destroys Goodman Brown's innocence.
The dark figure shows Goodman Brown and his wife Faith that all those people that they revered as godly and pure, like the town's Deacon and Goody Cloyse, a respected and pious woman, were present in the ceremony.
It reveals several sins and dark deeds these people have commited, allowing Goodman Brown to finally see the sinning nature of humanity. He renounces his naïve ideas about goodness and, now that he has seen the evil in man, he cannot trust anyone fully ever again.
