Genesis 1:14-18: The Purpose of the Lights of Heaven
God said, let there be lights in the expanse of the heaven to separate between the day and between the night, and let them be for signs and for seasons, and for days and years. And let them be for lights in the expanse of the heaven to give light on the earth. It was so.
God made the two great lights — the great light to rule the day, and the small light to rule the night — and [he made] the stars. God set them in the expanse of heaven to give light on the earth, and to rule over the day and over the night, and to separate the light from the darkness. God saw that it was good. (Genesis 1:14-18, my translation)
Why did God create the Sun, the Moon, and the stars? Here, we look at the reasons outlined in Genesis 1. The reasons could be summed up in three points:
(1) To make a (clearer) distinction between light and darkness, and thus between the day-time and night-time portions of a daily cycle.
(2) To provide lighting levels and effects suitable for day-time and for night-time.
(3) To provide celestial aids to aid humanity in tracking time and important events.
Regarding point (1): Since the sun, moon, and stars are created on Day 4 of creation, we must conclude that "evening and morning" were already distinquishable on some level. Evidently a light source and a rotating earth was sufficient for the first three days (see verses 3-5). But certainly the presence of the sun, moon, and stars makes the experience of day and night, light and dark, much more concrete and natural.
Regarding point (2): All humanity can easily see God's intention for giving the day and night cycles. During the day-time the light is (generally speaking) bright and shining, suitable for work, travel, and interaction. During the night-time, the light becomes dim and mellow, suitable for rest and for sleep, while still providing enough light for necessary activities. The shimmering vista of the moon and stars also provides a suitable backdrop for spiritual rest, giving us an opportunity to reflect on the day, and God's power and glory (cf. Psalm 19).
Regarding point (3): We have some difficulties to address, as far as what is meant exactly by "for signs and for seasons, and for days and years", and in particular what is meant by "signs". I have added an excursus at the end of this document, to explore those questions. For now, though, I will just say that the celestial bodies provide humanity with useful and universal aids in tracking time, such as the daily cycle, the lunar cycle, and the solar cycle. They aid also in tracking seasons of the year and important events. The Jews certainly would have been thinking of the religious festivals given to them later by God, which were tied into the agricultural seasons and to the lunar cycle.
I would like to add here that the study of the movement of the heavenly bodies is something which has fascinated great minds throughout all history. There is a quality and precision to their orbits and positioning which is almost musical in nature. For an intriguing and well-illustrated introduction to the subject, I would recommend Book VI in that popular volume entitled "The Quadrivium" published by Bloomsbury USA.
1. Meditate on the skill and power of God displayed by the sun, moon, and stars (Psalm 19).
2. God, at creation, desired there to be a distinction between the light and the darkness. God also desires there to be a distinction between spiritual darkness and spiritual light, that is, between good and evil, and between the children of darkness and the children of light (see 1 Thessalonians 5:5-7). If we are children of the light, let us not live godless lives after the pattern of those who do not know Christ.
3. In Genesis 1, and in the Hebrew way of thinking, a day cycle is made up of first an "evening" (עֶרֶב, ˓e-REV) and then a "morning" (בֹּקֶר, bō-QER), a period of dim, cool light, followed by a period of glorious brightness. Likewise, we live now in a time of spiritual twilight, in which there is much that may be known about God through his Word and the Gospel, but there is also much spiritual darkness and wickedness in the world around us. But the day of glorious brightness is coming soon, when Christ will return, and "the earth will be filled with the knowledge of the glory of the Lord as the waters cover the sea" (Habakkuk 2:14, ESV, cf. Isaiah 11:9).
For you are all children of light, children of the day. We are not of the night or of the darkness. So then let us not sleep, as others do, but let us keep awake and be sober. For those who sleep, sleep at night, and those who get drunk, are drunk at night. But since we belong to the day, let us be sober, having put on the breastplate of faith and love, and for a helmet the hope of salvation. (1 Thessalonians 5:5-8, ESV)
Excursus: For signs and for seasons, etc.
The Traditional Translation
This Hebrew phrase uses a mixture of the conjunction וְ (vav) and the preposition לְ (lamed). The traditional and straightforward approach, adopted by many translations, is to translate וְ as "and" and לְ as "for". Here is the Hebrew and English laid out in the ESV's interlinear translation:
וְהָי֤וּ (and-let-them-be) לְאֹתֹת֙ (for-signs) וּלְמ֣וֹעֲדִ֔ים (and-for-appointed-times) וּלְיָמִ֖ים (and-for-days) וְשָׁנִֽים (and-years)
With this approach, a natural question for the modern reader is what is meant by "for signs" (לְאֹתֹת, a form of אוֹת, ˒ōt). HALOT, a modern lexicon, gives many senses for this word, summarized here: (a) a distinguishing mark (b) a standard (c) a sign, as a reminder of duty (d) a commemorative token (e) a sign confirming the truth of an earlier statement (f) a miraculous sign, which proves someone's power or authority (g) an omen, fortelling of the future (h) a warning sign, deterrent.
HALOT lists Genesis 1:15 as an example of sense (g) "an omen". This is an interesting idea to ponder, considering that the Magi were guided by a star to seek the newborn Christ. But it is also difficult to reconcile with God's condemnation of astrology (Jeremiah 10:2).
The Amplified Bible gives an interesting interpretation:
"let them be useful for signs (tokens) [of God’s provident care], and for marking seasons, days, and years"
So, perhaps then the sun, moon, and stars are "signs" in the sense of being marks or tokens of God's power or goodness.
Another question, not quite as difficult, is what exactly is meant by "seasons" (וּלְמ֣וֹעֲדִ֔ים, a form of מוֹעֵד, mō-˓ĒD). HALOT lists this instance under sense 4 of מוֹעֵד, to mean "festival" or "time of festivity". The older lexicon BD provides a more general meaning of "a set time or season; especially festival days". So, it seems that the sun, moon, and stars are made available as aids in determining the start and end of special holidays.
The NET Translation
About half-a-dozen modern translations give something similar to
and let them be signs to indicate seasons and days and years (NET Bible)
The NET Bible's helpful translation notes explain:
The text has "for signs and for seasons and for days and years." It seems likely from the meanings of the words involved that "signs" is the main idea, followed by two categories, "seasons" and "days and years." This is the simplest explanation, and one that matches vv. 11-13. It could even be rendered "signs for the fixed seasons, that is [explicative vav (ו)] days and years." [...] The point is that the sun and the moon were important to fix the days for the seasonal celebrations for the worshipping community.
The great thing about this translation, is it presents the sun, moon, and stars as signs, or indicators, of the other things mentioned in the verse — seasons, days, years. So you don't have to struggle with a separate question of to what "signs" the phrase is referring. And it keeps the phrase contained to presenting one basic idea — the celestial bodies as indicators of time.
The downside, for those who know a little Hebrew, is that it doesn't give a clear explanation of the first vav (ו) in the phrase. As acknowledged by the NET Bible notes, the text literally reads "for signs AND for seasons". Is that vav also supposed to be Explicative, or some other specialized use of the vav...?
 Quadrivium: The Four Classical Liberal Arts of Number, Geometry, Music, & Cosmology, ISBN 978-0-8027-7813-0. This hard-cover book is actually a collection of six books. The sixth book in that collection is "A Little Book of Coincidence in the Solar System" by John Martineau.
 Cf. ESV, NASB, KJV, NKJV, NHEB, WEB, ASV, Darby, Geneva, Bishops, Douay-Rheims
 Cf. NLT, BSB, CSB, ISV, NET Bible, CPDV, Alter
 In Hebrew, the conjunction וְ (not to be confused with the verb prefix) is usually Conjunctive (translated "and") but has some other specialized uses including the Explicative וְ (translated "namely, even, specifically"); the וְ in hendiadys; and the rare Pleonastic וְ (a superfluous vav included for purely stylistic reasons). See Williams §430a, §430b, §434, and §435.