From these verses in Bereishis Chapter One and Rashi's explanation we can learn an amazing thing about the purpose of the universe.

14. And God said, "Let there be luminaries in the expanse of the heavens, to separate between the day and between the night, and they shall be for signs and for appointed seasons and for days and years.

Rashi: and they shall be for signs: When the luminaries are eclipsed, it is an unfavorable omen for the world.

15. And they shall be for luminaries in the expanse of the heavens to shed light upon the earth." And it was so.

Rashi: And they shall be for luminaries: In addition, they will serve in this function, viz. to shed light upon the world.

Rav Chaim Shmulevitz zal explains that from both the order of the verses and the language of Rashi we can see that the primary reason for the creation of luminaries is make us aware when there is an unfavorable omen for the world. The light and heat that are absolutely essential for our existence are only secondary.

The main reason the sun and the moon were created is to inform us when our actions are not pleasing before G-d. They are there to tell us to introspect and search for straight path in life. This is an amazing to thought that needs to be contemplated.