Adam was the first and only man God created. He could not have expected to know automatically what he was supposed to do. So the Lord God had to give him instructions. He told Adam and Eve: God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish in the sea and the birds in the sky and over every living creature that moves on the ground.”
These words express God's original intent. They show us His original purpose for the planet that we call earth with words both simple and complicated at the same time: simple, in that He wanted to establish His Kingdom rule on earth; complicated, in that He created such a profusion of life that managing it (as Adam found out a little later) was not going to be easy. God has given a big responsibility to a very fallible being.