Because this book is basically about God's two Creations i.e. the Prehistoric creation and the new creation in man the reader might expect it to begin at Gen. 1. Right? No, very wrong. That would be like buying a newly built expensive house without knowing anything about the builder. So we begin with God, the maker of all things.
It is well to begin with God, because He is as unknown today as when Paul noticed an inscription in Athens "To the unknown God." Even Christians, when asked to define the meaning of the word GOD are sometimes nonplussed.
Part I then was written to acquaint us with God in a broad sense, before we begin to consider His works. It has only two chapters. One is to explain who God is, and the other why He is unknown in the world today. If you are interested in God, and the associated question of God and man, read on.