The sleeper dikes are usually located at places where there is a high risk of dike breakthrough or where there used to be such a risk. The dike is only potentially safeguarding the land from the sea/river, and is "sleeping", hence the name. Sometimes there is even a secondary sleeper dike, which can be called a "dreamer dike".
Many of the sleeper dikes are historical, and have after the dike improvements of the Delta Plan no longer a theoretical function in protection against the water. However, in case of a breakthrough they do divide up the land in sections, and therefore stop the waterflow in some parts, and slow down in others. Also many of these dikes are protected because of their cultural historical and ecological value.