Automatic, as its name suggests, automatically recalculated your route, should you venture off it. This works well if:
A. You have simply asked your dumb (clever) device to take you from A to B, down roads of its choosing, selected by the device according to your preference settings. In short it will, no matter what you do or where you go, always take you to point B. This is good enough for many users.
B. You create your own route A to B, via assorted via points or waypoints ie. announced points that you have told the device you must pass through. You dumb (clever) device will route you through them all without fail, unless you use the skip button. This is good enough for some or many users.
It has other uses, but for these (and indeed off and prompted) you need to understand how recalculation works on the dumb (clever) device.