If you are unsure what the purpose of a basemap is for Garmin navigation devices this page should shed some light on the subject.
So then, what is a garmin basemap and do you understand basemaps?
Basemaps are loaded on Garmin Devices in order to provide a bare minimum map on the device. They normally contain major roads only and are not suitable for detailed routing. However they are very useful overall.
The basemap is stored on the unit under drive:/garmin/ as gmapbmap.img. You can see your basemap details by visiting, system, about. The display will look something like this.
The basemap cannot be unselected like normal maps (neither can the oem maps fyi). The main advantage of having a basemap is that once you pass a certain zoom level the basemap is used instead of your detailed maps which allows for quicker drawing on the device. This Screenshot shows the basemap being used instead of detailed maps at high levels.
As you zoom out further you will notice that the grids for you detailed map overlays appear on the basemap making is easy to see which areas you have detailed maps for.
You will notice that areas without detailed maps are still visible thanks the basemap.
There you have it in short a basemap is the underlying map on a garmin device which is used at high zoom levels and in areas for which you may not have detailed maps.