Density depending on temperature

Bild Chart: Density-temperature structure of water compared with benzol; comparison shows the density anomaly of water. Most substances have a continuous trend: the higher the temperature, the lower its density. In phase transitions (gas –> liquid –> solid) the density changes drastically in this temperature range. Water however has a density change at its freezing point. The density does not increase as one would generally expect but drops. The graphic shows the density-temperature behavior of "normal" substances (here benzol) as a contrast. Information and ideas:What could cause this behavior?What practical importance does this anomaly of water have?