The purpose of "extras" is to allow the leader to share or bring to light other observations from the text that are interesting, fascinating or thought-provoking, but not necessarily relevant to the main point. These "extras" are usually the result of the leader's study and research and are a subtle way of encouraging the group to look deeper into Scripture in the course of their reading.
This can be a very fun and entertaining conclusion to the study—or even introduction—as the leader knows the answers to the "mystery" question and the members have to try to figure them out.
As an example referring to 1 Samuel 17:40, the leader might ask, “Why do you think David picked up FIVE smooth stones when he only needed ONE to defeat Goliath?” (Possibly to kill Goliath and all his brothers in anticipation that they might come to avenge Goliath’s death.)
This is positive reinforcement to the group that the observation part of studying Scripture is often critical to making the right interpretation and personal application.