Genesis 1:18-19

to rule over the day and over the night, and to separate the light from the darkness. And God saw that it was good. And there was evening and there was morning, the fourth day.
(ESV)

Here we have the purpose of the lights laid out with a little more detail. The first purpose is to rule the day and the night. Obviously talking about the sun and the moon primarily. The second purpose is to separate the light from the darkness. This is something that God had already done on a previous day, but now he is handing that over to the physical sun. There had already been day and night for three days without the help of the sun, but God had now given this heavenly object the role of bringing light to the day. This role was already mentioned in verse 14.

Another role mentioned in verse 14 pertains to giving signs and seasons, and days and years. The stars and the sun and the moon fulfill that role perfectly. So perfectly and so precisely that there are whole sciences dedicated to studying the movements of these heavenly bodies. There are millions of dollars poured into the study and discovery of new things out there. These things that ultimately were made just for bringing light to the earth.

Notice that, once again, these things are all good. God is somehow able to view the whole of the universe, that vast expanse, and state, "It is good!" I wonder, did He boom that voice across the universe when he said it?

By the way. This means that the oldest molecules in the universe can be found right here on this planet. This little rock of a planet is the oldest thing out there by three days. How is it possible that the light has made it from those other stars and galaxies? C'mon! He's God! He just spoke a Universe into existence! He just imagined and spoke every DNA sequence for every tree and every plant into existence! A question like, how did the light get here, seems like a moot point!