what's behind the wall?

At first, we were very puzzled to see many empty lands with high concrete walls and big gates on the perimeter of these plots of land. There is NOTHING behind the walls. I suspect the reason is for the land owner to protect his land. Slums emerging overnight on empty land is not surprising and removing the slum occupants may be difficult, time consuming and costly. Therefore, the first thing a land owner builds after the purchase of his land is high concrete walls with big gate. This prevent unwanted slums to settle on the land.