Genesis 1:14

And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:

1. God declared his [I'm continuing to use the pronoun "his" for convenience, even though I don't think the single occurrence of a male pronoun so far is sufficient proof that God is male.] intention to create the sun, moon, stars and other light-emitting or reflecting heavenly bodies.
2. The sun, moon and stars can be used to measure parts of days, days, seasons and years.
3. Some things involving heavenly bodies may actually be signs.
4. God thought it was important to divide time into various segments. I wonder what it would be like to live without any of the time divisions to which we are accustomed?