Without knowing both sides of a story, you cannot make a fair comment on that story or pass a fair judgment. This is one of the first things that students in journalism school are taught. It's also why every trial consists of both prosecution and defense teams.
For example, let us consider a fictitious case in which a young adult is found to have murdered his father. Let's say the murder was witnessed, and the witness came forward and said that he saw the man put a gun to the older man's head and pull the trigger.
It may seem like a cut-and-dried case until police speak to the accused and learn that the entire family has been subjected to years of physical abuse at the hands of the deceased father. While the son is still guilty of murder, there are now extenuating circumstances which may have an impact on his conviction or sentence.
Without hearing both sides of the story, this would never have come to light.
I remember being taught in college that there are always three sides to every story: Person A's account, Person B's account, and the truth. Only by hearing both accounts can an impartial observer hope to arrive at the truth.