Why did God create the stars, sun, moon, and other planets?
"The Bible says that the stars, sun, moon, and planets were created for us---they have a purpose---they did not randomly evolve"

1) To serve as signs to mark the seasons, days, and years [Genesis 1:14] for life here on earth
2) To give light upon the earth [Genesis 1:15, 17]
3) To "govern" the day and the night [Genesis 1:18]--to provide consistent markers, giving order to creation
4) To separate light from darkness [Genesis 1:18]--to show visually here on earth, the distinction between the two
5) To declare the glory of God [Psalm 19:1-6]
6) To cause us to praise Him [Psalm 136:3-9]
7) To reveal His attributes to mankind, and to draw mankind to call upon Him [Romans 1:20; 10:18]

Serving as signs to mark the seasons...
It is easy to see how the moon and the sun have always been used for timekeeping. God provided man with these reliable timekeepers long before we developed timepieces of our own. Even today, time is based upon the "solar calendar". Our "months" originated from the phases of the moon..."the lunar calendar". This is why the Bible gives special attention to the sun and the moon as being the "two great lights". Migration for many animals is initiated by the change in the length of the day. So here the sun marks the seasons.

Serving as "governors" of the day and night...
The stars, planets, sun and moon also "govern" the day and night according to the Bible. What does that mean? By "governing" the day and night, these heavenly lights serve as consistent markers and guideposts for orienting us not only in time, but in place. Many birds use the stars as directional navigators for their migration. Sailors used the sun, moon, and stars for navigational directions. Even our own complex inertial guidance systems for air flight and space travel use the positions of the sun and stars to stay on course. Voyager 2 is an example of this. Voyager 2 used planetary positions and star positions to track its course. Without these heavenly bodies, man would be disoriented not only to TIME, but to SPACE as well.

Giving light...
Without the sun's light, we could not exist. But even at nighttime, the moon reflects the sun's rays to cast light upon the earth. The stars also shed a considerable amount of light upon the earth. These lights mark out day and night, without leaving the night utterly dark

Declaring glory...
The universe declares the glory of God much the way a painting declares the talent of an artist.
Louis Pasteur said, "The more I study nature, the more I stand amazed at the work of the Creator."

The other planets stabilize earth's orbit
and therefore promote life on earth:
The other planets, and also the stars, serve as gravitational and magnetic stabilizers to maintain earth's stable orbit around the sun. Imagine the sun, mercury, and venus pulling the earth. Because of the location of earth's orbit, it would tend to decay inward--towards the sun--resulting in extreme heat and an inability to support life. But God created Mars, Jupiter, Saturn, Uranus, Neptune and Pluto to provide a stabilizing pull outward to keep the earth in its orbit. Likewise, if Mercury and Venus were not there, the earth's orbit would tend to decay outward--resulting in extreme cold and lack of life on earth. So the planets [and even the stars!] provide a stabilizing effect upon earth, so that it can maintain life!!!
This stabilization was described by...

Pierre-Simon marquis de Laplace [March 23, 1749 --March 5, 1827]
French mathematician, astronomer, and physicist.
Atheist
He is best known for his investigations into the stability of the solar system and the theory of magnetic, electrical, and heat wave propagation. In his major lifework he applied Newtonian gravitational theory to the solar system to explain deviations of the planets from the orbits predicted by the theory (1773). Laplace established a mathematical basis for the stabilization of the solar system. He is well known for the LaPlace Equation, which describes this phenomenon.