Sforno on Genesis 1:18/ספורנו על בראשית א׳:י״ח

1ולמשול ביום ובלילה, in order to bring new creatures into life in the “lower” regions of the universe, i.e. these living creatures, as opposed to the plants, required more than the light G’d had created on the first day. Seeing they are more important and more sophisticated beings they require exposure to better light in order to flourish.

2ולהבדיל בין האור ובין החושך, to establish clear demarcations amongst the inhabitants of the lower regions of the universe, between when these respective luminaries were to shine, and when they were to set, i.e. to disappear from view. Basically, the meaning is the same as in verse 14 above.