The dead lifeless planets demonstrate the specialness of the earth, like Goldilocks porridge it's not too hot, not too cold, but just right!
Read yer bible and see how God 'selects' things and people for His purpose. Genesis 1:1 shows that of all the created universe God selected the earth for a special purpose. Even science has determined that the earth is incredibly unique in all the universe (their athiesm notwithstanding ). God then created all sorts of critters, and selected man out of them for His purpose. Of all the critters he made only a select few are useful to man. He then produced all sorts of people, but selected but a few out of them for His purpose. It seems that God wants all to see the contrast or difference between the things He creates generally, and the things He selects, or 'sanctifies' (sets apart for a holy or special purpose), specifically. So rather than question the purpose of the other dead, 'unselected' planets, think about how wondrous it is to be a part of the specialness of earth and everything about it.