The advantage of using warm water is that water holds much more heat than almost any other commonly available non-toxic substance. This means that it is capable of thawing a frozen item quickly without overheating it. Because the multiplication of bacteria in the food as it is thawing is a function of time as well as temperature, the more quickly you can defrost a food item to optimum feeding temperature (without damaging it - so stay away from the microwave) , the fewer bacteria there will be.