What caused the Dutch to build windmills throughout the flatlands?

Industrialisation was the main cause. Windmills were primarily used for corn miling, land drainage and saw miling. Since the Dutch experience problems caused by wind and sea, they tried to make good use of their causes. So they built windmills throughout the flatland.

Geographically, windmills marked places in the flatlands when there was no such thing as GPS or maps.