God created the heavens and earth.  Then, something happened.  The dinosauric ages ended, but water life continued in the gap between dinosaurs and mammals. Then, God decided to REPLENISH the earth, so He said, "Let there be light."  The word "replenish" means to re-fill.  Re--plenish. Which implies that there was something here on land before this.  He didn't need to replenish the water life.

Three words in the first chapter of Genesis are important:  1. Create, 2. Made, and 3. Let.
1.   "Create" means that you design a brand new thing (that is perfect) out of nothing.
2.   "Made, or Make" means that you take something that is already here and turn it into something  else.  (I'll get some fabric and "make" a dress.)
3.   "Let" means to not stand in the way of something occurring.  (I'm going to open the blinds and "let" the sunshine in.)

The word "Create" is used four ways in the Old Testament.  1. God created the heavens and earth.  2. He created large animals, behemoths.  3. God created man.  4. Then later in the Psalms, when David said, "Create in me a new heart."  (David knew that he could be righteous only if he became something entirely different than the sinful man he was.  It had to be a creative act of God.)

The progression after God said, "LET there be light" was orderly.
1.   Light had to come first.  You can't Re--plenish without light.  God parted the dust clouds.
2.   Division of the light from the darkness. (Rotation of the earth) We have no idea how fast or slow rotation was.  Rotation time, even today, changes.  The length of a day back then is uncertain.
3.   Separation of the waters above the earth from those below.  Rain, clouds, and such.
4.   Separation of the water on earth from land. Oceans, lakes, rivers, and streams--and earth
5.   Finally the earth was ready for plant life to re-emerge. God said, "Let the earth bring forth grass, the herb yielding seed, and the fruit tree yielding fruit after his kind, whose seed is in itself..."   He is getting ready to end the "Gap."  God is putting things in order for land life to survive.  This progression of "days" is a preparation for life.  These days depended on how quick a rotation of the earth was.  Even science--when they talk about time--is concerned with how long a day, or a year was.  We know from scientific records that an earthquake or volcano or meteor strike can speed up or slow down a rotation.  Continued.....