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 and it can be used at high zoom levels and in areas for which you may not have detailed maps.
A basemap is to allow browsing big areas. detailed maps aren't prepared for this task, with big zooms, device switches to basemap. without it everything will work very slow.
DEM basemaps are bigger because they contains shadings, many users use new basemaps but with DEM removed with GmapTool ,eg. old Garmin units and Phones
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