One reason to challenge the traditional view of God is the claim that if He was an all-powerful, all-loving God, he would have made the world a better place, without suffering. Here we are looking at the response to this - the idea that this world is the best that it could possibly be. If God took away suffering, it would actually make things worse.
Many people think that this is not the best possible world because there is still a lot of suffering that is not neccessary. There are still people starving in different countries and there are things which we could do to stop this! Therefore this is not the best possible world. People are ill and dying from diseases that would not be circulating if the conditions were improved. Therefore this is not neccessary and this means that it is not good/the best possible world. If people would help other countries, the world would become a better place.