What is the significance of the dead cities in "The Million Year Picnic?" What is their symbolic or metaphorical importance?

"The Million Year Picnic" is the final part of Ray Bradbury's interconnected series of stories The Martian Chronicles. After the outbreak of nuclear war on Earth, almost all of the Martian colonists had returned home to try to help the relatives and friends they had left behind. This had left the numerous cities that they had built on Mars deserted.
A family consisting of a father, mother, and three boys load a rocket with supplies and escape to Mars from the disaster that has befallen Earth. They hope to meet up with a few other families, repopulate Mars, and start a new, more peaceful society. The father says of the old life on Earth:

"But that way of life proved itself wrong and strangled itself with its own hands."

The old empty cities on Mars have two symbolic meanings that are almost the opposite of each other. They symbolize the wrongness and emptiness of the society of the past that caused such massive destruction and loss of life. The father symbolically breaks the family's association with that society when he burns all the documents from the past. He says:

"I'm burning a way of life, just like that way of life is being burned clean of Earth right now."

The empty cities also symbolize the new hope that the family has come all the way to Mars to find. The cities belong to them now. They have not just the cities but the entire planet on which to construct a new and better society. As the father points out at the end of the story, the world is theirs now. They are the Martians.