Genesis chapter one is all about purpose.
“And God said, ‘Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.’ And God made the expanse and separated the waters that were under the expanse from the waters that above the expanse. And it was so.”
So God says the expanse – the sky – is necessary to separate the waters. That’s its purpose, its job, and it’s a job fundamental to its essence.
What about the trees and the plants? They have a unique and essential purpose too: