"A basemap is a non-editable layer that provides background, or reference information, in your map. It is typically designed to provide a visual reference for other layers to help orient the user of the map. Aerial imagery, terrain, and streets are examples of basemap layers."
In other words: this file is an image of the Earth; when we have a "ZoomOut", instead of maps the software use basemap file. It does not influence the routing process, just arrange the graphics.
In conclusion: always use basemap file with any set / combination of maps; preferably to use the latest released (it should be most accurate - as Midnight Blues just mentioned) .