Why were the Traction Cities initially seen as useful and essential in the novel, Mortal Engines?

The Traction Cities were seen as useful "a thousand years ago when there were all those earthquakes and volcanoes and the glaciers pushing south" (42). They were necessary when humans had caused the earth to become volatile, and they had to move constantly for survival. Now that the earth's weather has become more stable, however, they have lost their usefulness.