Because the jury may not get the full story. If someone just says 'this person is accused of stealing this bread, but he says he didn't.'
Even if two people give as detailed an account as they can, they're going to be different and contradictory, so it becomes he said, she said sort of thing.
Lawyers are supposed to take those stories, find others who saw parts of that story, use evidence gathered, and to basically make all the facts with the case as clear as day so the jury will make the right decision.