Hai Vincent,
The nice thing about a build-in map is that you can recognize where you are in a glance.
The base-map build in my GPSIII covers the entiere world. It show's citty's, lake's, rivers, main-road's, rail-road's. In this you see a little arow, that's you. In this way it's easy to see if you are near that town already, or maybe you missed that junction. 
Also it's easyer to "project" your position on a (paper) map. Even if a map has coordinate's (witch a lot do not  ) it's usually in a very rough scale (every 10 degree's) while you want your position as clearly as possible (want to find that well in the dessert?).
I hope this clearafy's the ease of a build-in map.
Maarten
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