God created the earth to be inhabited by humans. His original purpose was that they live forever in a paradise earth, with perfect health, and that they be at peace with the animals. Isaiah 45:18 says "For this is what Jehovah has said, the Creator of the heavens, He the [true] God, the Former of the earth and the Maker of it, He the One who firmly established it, who did not create it simply for nothing, who formed it even to be inhabited..."
As our heavenly creator and father, he deserves our worship and obedience, and there can be no greater purpose in life.
I was told that we each have a purpose / job to do. Some people spend their whole lives searching for what they are supposed to do and die never knowing. I have a friend that I think was put on this earth to teach me patience. Does that count?