God's reasons are given in Genesis chapter 1, verse 14: To help man divide day from night, to be for signs, and so we can keep track of time.Originally Posted by berrywise
But there is also a more important reason given by God:
"The heavens declare the glory of God, and the firmament showeth his handiwork. Day unto day uttereth speech, and night unto night showeth knowledge. There is no speech nor language, where their voice is not heard." Psalm 19:1
In other words, this enourmous universe was created by God to tell you and every other human on earth "I exist."
Even when I was an unbeliever, I would often marvel at the universe and the utter incomprehensible size of it. Now I marvel at the God who made it.