Because prior to the introduction of light, "darkness was upon the face of the deep," one would assume that darkness was created before light.
I assume it was created because nothing supersedes God, Who created the world from nothing, whereas darkness is most certainly NOT nothing. In order for there to be darkness as we know it, space absolutely has to exist, and it would be wrong to assume that darkness is a "natural" characteristic of space, since nothing is "natural" unless and until God says it is.
Physical darkness is the absence of the electromagnetic wave spectrum that the human eye (and any machinery we can conceivably invent) can detect.
Spiritual darkness is the inability of one's heart and/or soul to detect spiritual light. So far so good.
However, separating one from the other means - or does it? - that the two (light and darkness) overlapped, or were somehow merged together, before God divided them.
(Note also that He does not TELL them to go their separate ways; He divides them Himself, directly. Manually, so to speak).
What did the Universe in which light and darkness were mixed together look like? Why did God find it necessary (or did He?) to separate one from the other?
Gen 1:4 - And God saw that the light was good, and He separated the light from the darkness.

