This recognition of the signs showed some of the people their need to repent. Mormon is repeating this benefit of the sign because he wants to emphasize the miraculous nature of it. It was a miraculous occurrence, but even more, it had a miraculous effect in the lives of many. As we will see, Mormon is not telling the entire story, as he makes it appear as though all of the Nephites saw and were converted. Shortly his narrative will make it clear that there remained many who were unbelievers.