A low water cut-off is used to shut off the boiler in case the water level drops below a safe level. This is important if there are portions of the boiler system below the level of the boiler as you may not notice the effects of a leak before the boiler is dry, which could lead to significant damage.
If the boiler is the lowest point of the heating system you will likely have the time to spot and rectify any leaks before the boiler is empty, so a low water cut-off is less critical. In either situation it is a useful safety device and may be required by code.