think of it this way : have you ever as a child taken a jam jar, put some soil in it, some plants + a couple of grubs and beetles ? + then see how things change over time ?
that's what the earth is like : just some kid's jam jar half-forgotten on a shelf in the garage
the universe (i.e. the house) just happens to be where the kid lives, and was not built for the sake of the jam jar - it just happens to contain it
so much for the question why god would create a whole universe for the sake of a planet full of bipeds and other assorted organisms - the answer is, he didn't
