What do you think was Irving's purpose in writing this story?

Arguably, Irving wrote this story as a warning to his readers that living a life motivated by financial gain will only bring misery. He shows this clearly through the character of Tom Walker. When he makes a deal with the devil, he agrees to become a moneylender in return for the treasure.
Although Tom becomes a "rich and mighty man" in Boston, he begins to feel "anxious" about what will happen when he dies. He starts going to church as a means to save his soul, but his desire for wealth does not fade. He forecloses a mortgage on a poor "land jobber," for example, who begs him for mercy. Instead of helping the man avoid a life of poverty, Tom shows no compassion and goes ahead with the foreclosure. Shortly thereafter, the devil appears at Tom's door and takes him away on a horse. Tom is never seen or heard from again.
Through this ending, Irving argues that people should not live a life dedicated solely to the pursuit of financial gain. If they do, it is certain to bring ruin.