Just to put a finer point on things:
The reason high wing airplanes have a "both" setting is that fuel flows downhill. If one tank empties before the other, the remaining fuel from the other tank will gravity-feed and displace the air in the lines. So air is essentially never drawn down the fuel lines to the selector until you're really out of gas.
On a low wing aircraft, the fuel must be pulled uphill from the tanks to the selector and pump. With a "both" setting, as soon as one tank unports, it will start feeding air into the system. The fuel in the other tank will not be able to displace the air, since it nothing is driving the remaining fuel uphill against gravity.