Studies in Genesis 1
And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so. And God called the expanse Heaven. And there was evening and there was morning, the second day. (Genesis 1:8 ESV)
If we only count the words, not much happened on the second day of creation. God separated water and made an atmosphere. Though water is basic to life it does more than support life.
Water cleanses and forms. Running and flowing water washes away impurities. Yet, as flowing water moves it also forms and shapes. I imagine, and this is just my imagination, God uses the second day to begin forming and shaping Earth into a planet with a gravity strong enough to hold an atmosphere and not lose the newly formed water.
Evaporated water moves through the atmosphere picking up particles, forming clouds and rain. Each rain drop contains minute particles of dust with just the right molecular geometry for the drop to form and may become heavy enough to fall. God washed and cleansed the atmosphere.
Falling rain cultivates the ground, softening soil, preparing it for growth. God used water, integral to live, to prepare the Earth for life.
Water interacts with light. Even though we do not hear about rainbows until after the Flood in Genesis 9, God filled His creation with color. Flowing and moving water reflects and refracts the surrounding light. God creates beauty.
God finishes the second day of creation with the Earth ready for life.