I have two possible answers depending on your point of view on creation:

1: God loves to create. We love to create things and are made in his image. Look at nature; there is so much variety. All of creation including other galaxies ect declare his glory

2: Scientist have shown that the Big Bang (if you are not a creationist) needed to be finely tunes so that it would form matter. If it were smaller, then it would have collapsed in on itself with no matter formed. If it were bigger, then elementory particles would have been spread too thinly for gravity to bring them together into stars leading to planets ect. You could argue that nature doesn't set the rules God does, but perhaps it was the best method to achieve the environment on earth. A large universe was needed.

It is probably a combination of these two points.