The closest reason I've ever thought worth considering is entertainment. The angels were bored and implored god to devise some kind of diversion. So god made an ant hill, and, like a kid, from time to time dumped water on it, shoved it around a bit, and inflicted various types of suffering on the poor creatures The angles were delighted, and remain so.
That humans are entertainment makes most sense. That means anything created would be to please head hancho and anything else is just filler. Allows god to be completely soverign and the reason for anything to be is because god wanted it so. I often like the artist depiction, god some crazy artist showing us his passion through his work, still falls under entertainment.