This is when you only want to clean only left half of the room, for example. In the old robotics vacuums, your choices were either enough physical "virtual wall" units (those little stubby boxes you place on the floor) or physically fencing it, with overturned chairs or boxes or something.

With the smart vacuums, you get the nifty map and you can draw the lines/boxes around the area you want cleaned. Very convenient, but naturally requires some sort of device with a screen (to show map) and position input (to draw the boundaries), like a phone.