If they get nothing or a lump of coal they may play a practical joke on the people in the house.
This also started with the Celts. They believed that on Samhain some ghosts tried to return to their old homes.
People tried to get the visiting ghosts in a good mood by offering them nice food. They put fruit, nuts and treats on their doorsteps.
If the ghosts were angry they would mess up the farms and frighten the animals.