Especially in historical and biographical books when the narrative shifts to a new person, location, or event it is often an indication that the author is transitioning to a new theme or topic. The people and events often unlock the purpose of the text. Other genres may also make note of a person or event and when they do make note of it, but this is most common in the narrative portions of the Bible.
For example, in the Gospel of John, John often uses temporal and geographical changes to indicate a change in focus. These are good indicators of section divisions, theme changes, and changes in topic.