According to the author of the book of 2 Kings, this story shows how God kept a promise through the prophet Elisha and gave the woman a baby just like Elisha said. It teaches that God can do amazing things and answer prayers in wonderful ways.
2 Kings 4:17: And the woman conceived, and bare a son at that season that Elisha had said unto her, according to the time of life.
This verse suggests Sarah was past the age of childbearing and unable to have children, contradicting the miraculous conception in 2 Kings 4:17.
Genesis 18:11: Now Abraham and Sarah [were] old [and] well stricken in age; [and] it ceased to be with Sarah after the manner of women.