1. Temperature control: The time and temperature of steaming egg cakes need to be precisely controlled in order to avoid over-steaming or under-cooking the egg mixture. Warm water makes it easier to control the temperature to ensure the eggs are steamed at the right temperature.

2. Taste: Steaming egg cakes with warm water makes them more tender and soft. Cool water will cool the egg mixture, while hot water will cook the egg mixture too quickly, resulting in a hard texture.

In summary, you should use lukewarm water rather than cold or hot water when steaming egg cakes.