What are the advantages of anomalous expansion of water?

Normally, liquids contract on cooling & become more dense. However, water contracts when cooled to a temperature of 4oC and thereafter expands as it is cooled further from 4oC to 0oC. Water attains its maximum density at 4oC. This phenomenon is useful for the preservation of marine life in very cold temperatures. Initially, the surface water in water bodies starts cooling. Upon reaching the temperature of 4oC, the surface water descends to the bottom as it denser. Upon further cooling between 4-0 degrees C, a temperature gradient is set up in depths of the water body whereby, the bottom-most layer is at 4 deg C & the temperature gradually drops as one goes upwards. At 0 deg C, ice is formed. Ice being lighter than water, floats to the upper surface.and also because ice has space or kind of holes between it's molecules it is able to float in water. Further, water and ice are bad conductors of heat. All this help in the maintenance of temperature of the water at the bottom at 4 deg C. It is in this layer that marine life is able to sustain itself.