Ge 1:4 And God saw the light, that it was good; and God divided the light from the darkness.
5 God called the light Day, and the darkness He called Night. So the evening and the morning were the first day.

Why must there be light and darkness, day and night? God, in His wisdom, decided that man would be characterized by a period of activity and a period of inactivity.

Joh 9:4 "I must work the works of Him who sent Me while it is day; the night is coming when no one can work.

Daytime is work time. Night time is rest time. However, in the new heaven and the new earth, it is said that there will be no night there.

Re 22:5 There shall be no night there: They need no lamp nor light of the sun, for the Lord God gives them light. And they shall reign forever and ever

This is because, all who will be part of that new creation will be bustling with activity for God all the time, with their rejuvenated and resurrected and glorified bodies.

It appears that it was at this point that God formed the earth round. The expression "God divided the light from the darkness" seems to imply this. The division of light from darkness, day and night, is the result of the rotation of the spherical earth from the source of light.

It appears that the earth in its initial stage was not spherical. It was a fluid mass but it was not round to allow the division of light and darkness as we know today.

The expression "the evening and the morning were the first day" implies the 24-hour rotation of the now spherical earth with reference to the source of light.

The divine activity of the "first day" included the creation of the earth initially, light was made to appear and the division of light and darkness, day and night.