From the World Heritage inscription:
Schokland was a peninsula that by the 15th century had become an island. Occupied and then abandoned as the sea encroached, it had to be evacuated in 1859. But following the draining of the Zuider Zee, it has, since the 1940s, formed part of the land reclaimed from the sea. Schokland has vestiges of human habitation going back to prehistoric times. It symbolizes the heroic, age-old struggle of the people of the Netherlands against the encroachment of the waters.
If you visit Schokland, you will find a lighthouse and a harbor in the middle of a farm field. This is because it was an island prior to the sea around it be reclaimed. Of all the world heritage sites in the Netherlands, this is probably the most dubious to be on the list. It is seems to be of national importance to the Netherlands but not really of world importance. I think the “human habitation going back to prehistoric times” was just part of the PR campaign to get this included on the World Heritage list.