I've read that they exist because of sin.
When the world was created there was no need for man to work the ground. There was not rain as we know it, but mist that came up from the earth.
Because of man's sin, God said the earth would no longer freely yield it's goods to man, but man would have to work for it. The "shield", like a layer of mist that encircled the heavens around the earth was removed, opening it up for weather systems that would change the environment and cause the earth to have to be worked by man.
Chrys