The God of the Bible is a God of order, and his method of Creation depicts that. One motif of Genesis 1, following the initial creation ex nihilo (see the commentary on verse 1 for more on that), is him separating things out of the watery chaos described in verse 2 and then giving those things a name.
Targum Onkelos: And the Lord saw the light that it was good. And the Lord distinguished between the light and between the darkness
Rashi: And God saw the light that it was good, and God separated: Here too, we need the words of the Aggadah: He saw it that it was not proper for the wicked to use it; so He separated it for the righteous in the future. According to its simple meaning, explain it as follows: He saw it that it was good, and it was unseemly that it [light] and darkness should serve in confusion; so He established for this one its boundary by day, and for that one its boundary by night.
Rambam: God confirmed the light in its existence in God’s will. God set the measure of the light and of the darkness
Ibn Ezra: God understood that the elemental fire was good. God divided the light of the fire from the absence thereof