The supposition that what Nephi sealed was the rain from heaven is confirmed in verse 6: “[T]he earth was smitten that it was dry.” In the first year, when the drought began, there were probably already crops in the field and stores set aside. The yield might not have been as good, but the first year of a drought wouldn’t have been serious. However, in the second year, the lack of food became critical. It was also a widespread drought, so that even trade could not bring in food, because the possible trading partners didn’t have food either: “[T]he whole earth was smitten, even among the Lamanites as well as among the Nephites.”
Times of desperation awake more feelings of dependence upon God that do times of prosperity. These were times of desperation, and so the people began to ask Nephi to pray for relief.