In answering this question we first need to get rid of a fatal misunderstanding that people have about creation and God. The misunderstanding goes like this, when men make something it is usually because there is a need for that which has been created. For example, people needed more water to irrigate their farms with so they built a dam to keep the water from running away before they could use it. Now the thinking goes that since we build and create things because we have a need, then maybe God created the world because He had a need. When people think like this they are most often thinking that God was all alone and so had to create beings with which He could communicate and love. Fortunately this is not the case. For since before the beginning of time our God has communicated with His Son and with the Holy Spirit. Also the Bible teaches us that God does not need anything.

The Bible tells us that everything that God does is for His own glory. So He created the entire universe for His own glory. In addition to His glory, God created the earth so that people like you and I would have somewhere to live. In our solar system there is nowhere where humans can exist without special apparatus to keep us alive. 

So the answer is that God created the earth for mankind. This gives us the privilege of ruling over the earth and the responsibility of looking after it on God's behalf.