The correct answer to this open question is the following.
It is included because an inset map shows a close up section or an overview of an area.
The inset map is useful and has its purpose. It is a small map that is included in the same page of the main map, The scale is different and helps you to find details or information about the location. Cartographers use this inset map to highlight places that cannot be found on the general map. One example of this use can be found in ATlas Road Maps, where you can see the map of the state but also on the same page you will find inset maps of the main cities.
