A water bath helps maintain a constant temperature.  Water has a high "heat capacity," which is the amount of heat needed to increase the temperature of an object by 1 degree Celsius. The heat capacity of an object depends on the chemical makeup and mass of the substance. If an object has a higher heat capacity, it takes longer for its temperature to change. For example, aluminum has a heat capacity of 0.90 J/goC, while water has a heat capacity of 4.18J/goC. This means that aluminum's temperature will fluctuate with its surroundings over 4x faster than water. So, if you bake something in a water bath it helps maintain a constant temperature. Once the water reaches a warm temperature, if anything causes your oven to shift in temperature at all (e.g. you open the door to check the cake), the water bath will hold that temperature without changing. 2. Indirect source of heat = even baking  A water bath also delivers a more indirect source of heat to the flourless chocolate cake! This results in the cake being evenly baked. 3. A water bath adds moisture.  A water bath also adds moisture to the oven, keeping the flourless chocolate cake from drying out or cracking.