Why Did God Create Earth and Man?
Not being God, it's hard to know exactly why He created man. But based on scripture, there are some things we can gather about man's existance and the nature of God to come to a conclusion.
Earth is the easier of the two questions. Earth is a vehicle for man to exist. We can go through all the different things about evolution or what not, but the truth is that Earth is specifically designed for life. There are more comprehensive studies out there, but this gives you a pretty good summary about the arguments.
Now, the tougher question: Why did God create man? Some would say, "To worship Him". It is a reason man exists, but I don't feel that it is sufficient.
In Genesis 1, Moses doesn't talk about the creation of angels, but we know that they exist. So, sometime, between the first day and the seventh day, God created angels. If you look at the roles that angels play, they serve as messengers, soldiers, and worshippers.
If God wanted worshippers, He had them in the form of angels. Why then, would he want people to worship Him? Some say that he wanted a creature that could choose to worship him. Now here, you get into a little bit of tricky territory. If you ascribe to the theory that God created Satan, or Lucifer, as an angel who rebelled, then angels have that choice. If you ascribe to Satan and demons as independant of angels, then they have no choice. I ascribe to the former theory. This presupposes that angels had a choice and some chose to serve God and others chose not to. So in this category, angels and men have the choice to worship God or not.
So what makes man so special?
We don't know about the creation of angels, so we can't really talk about them anymore. But we do know that Man was created in God's image. What does that mean, exactly? This verse is often explained by C.S. Lewis in the following way (paraphrased):
When man makes a statue, he usually makes it in his image. It looks like a man, but the similarities end there. The statue has no life or intelligence. In a similar way, when God creates man in his image, we are a far removed creation of his.
Whether this means we physically look similar to God or have characteristics similar to God, I don't know. What we do know is that Man has a spirit or soul. Through original sin, Man's suffered spiritual death. This is not to be confused with the punishment of physical death.
Before Adam sinned, he was in communion with God. God talked with Adam and they had a good time. But God didn't need companionship. he already exists as a triune spirit: the Father, Son, and Holy Spirit. These three existed in perfect loving communion with each other. So this brings us back to why God created man.
Worship glorifies God. Worship through song, dance, poetry, or any other form of expression glorifies Him. Nature glorifies Him through its existance. God even glorifies Himself. Repeatedly, Jesus talks about how does what he does, not to glorify himself, but to give honor to his Father.
Just as everything else in Christianity, the crux is Jesus. Jesus incarnation glorifies the Father. Jesus obedience glorifies the Father. Jesus sacrifice through His death glorifies the Father. Jesus resurrection glorifies the Father. Could it be possible that Man exists only so that Jesus can be God incarnate?
Think about this for a minute. You have thousands of years worth of human history to prepare for a singular event in time. Throughout the Old Testament, you see God choose people based on their faith. First you have Abram, then you have Jacob, then you have David, then you have Jesus. Throughout the process, you see God refining and refining setting up the perfect opportunity for His Son to walk amongst us. All of history led up to this point. The wars and politics established the climate for Jesus's message to be heard and for Him to be crucified.
Yes, Jesus's death does substitute for those who repent and put their faith in Him, but it is so much more. Even if no one accepted Jesus as their savior, He still would've come and died. Yes, because he loves us, but more so because Jesus loves his Father. Jesus birth, life, death, and resurrection bring glory to the Father that no one can even begin to comprehend. It is an act that can never be reproduced.
Is it possible? Could Man's existence be so insignificant? I hardly call the incarnation of God insignificant.
Ultimately, only God knows why He created us. But it's still fun to think about.