A planet is the only possible place where intellectual life such as human beings can develop. Especially, the terrestrial planets mainly made of solid materials are ideal if the surface can sustain water and air.
The vast diversity of planets in our solar system shows that such an ideal condition is not common at all. As the Unification Thought teaches, the entire universe including the solar system is designed to create an ideal environment for human beings to live. The arrangement, sizes, compositions, etc. of all the planets in the solar system are destined to be that way for such ultimate purpose.
In addition to the above practical purpose for humanity to develop and sustain its life, planets are there to give them knowledge about the origin and formation of the solar system for the better understanding of Earth.
Extrasolar systems serve such purpose, too. Every planet in our solar system serves the whole purpose in such a way. It is up to us, human beings, how we derive such knowledge from them and utilize them in the proper, original way of God's creation.