Basically the outside temperature may still not be quite as cold as your freezer, so it you turned off the freezer and left things to nature, your food may not be quite as cold or keep so well/long as it would normally.

That said, if the freezer is in a cold environment its rate of heat gain (thermal leakage through the wall-insulation) will be very low, and heat-pumps (which is essentially what a fridge mechanism is) are more efficient as the temperature-difference falls... so your fridge/freezer will use way less energy than usual if simply left to its own devices, if it's in a cold place already.