The Bible never explains why god decided to create the universe, nor why he decided to create humans.
JWs believe that god does not need anything, as he is complete and perfect. Hence, there doesn't seem to be a reason for the creation of anything. Which should leave us a bit worried. A being of incomprehensible power creates a universe and populates a vanishingly small speck of it with creatures that have spent almost their entire existence suffering. He arranges for a relatively small number of them to eventually have a blissful afterlife, while condemning most of them to eternal suffering.
So, why did he create us? Because he enjoys watching the suffering of others, seems to be the most logical explanation.