I don't know if you're equating Nametables with my Map Viewer idea, but such a viewer is always limited to the small portion of the map actually loaded in video memory (please my wrong use of technical terms here) and as you said, parts are not always in order.
It is useful however when there are small areas off-screen (a few tiles in any direction) that you can't reach otherwise, or to capture the actual map's graphic data (even if it's not in order) before various raster effects alter it too much such as per line palette shift and parallax scrolling, effects that more graphically intensive games tend to use a lot.