Inserts will separate out parts of a story from each other. No, not every book gets these separations and probably fewer books should.
I have two.
The first pictured would be Life as a Ghost. This story follows a young girl that fades into the background of life from high school, into college and through love. So part one is high school. Part two is college and part three is the acceptance of love. Since there were three distinct separations there are three distinct parts.
The second pictured, and the remaining images, are from Try Me. This was the first book I completed that had different sections. The first, Rosie, is told solelyfrom her point of view. The second, it's all Doyle's point of view of the situation. The third they take turns telling the rest of the story.