If all morals are subjective, then there is no absolute oughtness to how things should be. One person’s judgment on what is right or wrong doesn’t have to be another’s and neither has any absolute weight over the other. If someone wants to torture babies, it is only a personal value judgment. If justice was not brought to the Nazis involved in the Holocaust, then there’s nothing morally wrong with that. If one wants to be unfaithful to their spouse, then one doesn’t need to feel any guilt in it. If I want to cut in line in front of someone else, I can do so with impunity since I can simply believe it’s not wrong.
Further, if all morals are subjective, any moral argument against God fails. Why should anybody care what atheists claim about the morality of God if it’s just their personal judgment? I can then likewise believe all of God’s actions are good and neither position has any weight over another. It’d just be personal preferences.