Do the forward and afterward lend credibility to the story of "Rip Van Winkle"? Why?
In one sense, "Rip Van Winkle" by Washington Irving is obviously pure fiction, and not meant to be understood as a real history. However, such framing devices are common elements of fiction of the period, and especially of Gothic fiction, and thus signal to the reader two things, first, that the story is fiction with fantastic elements and second that the fantastic elements are being handled with a certain ironic distancing.
By framing the tale as part of a collection of folklore, Irving makes the unrealistic element of Winkle's long sleep less a jarringly unrealistic premise and more the sort of parable that we find in legends and myths. While Irving is not trying to convince us that the story is true, in the sense of having actually happened, the framing notes tell us that we should not read it as a realistic story of someone's life, but a fictional device used to explore the way the area had changed due to the American Revolution.