In any court case it is up to the jury to decide the outcome based on the evidence that is presented. If attorneys simply talked about the evidence it would not weigh much on the minds of the jurors. When an expert witness takes the stand and explains what the evidence is as well as why it is relevant, the jurors usually consider the evidence more reliable. An expert can present the evidence in a way that is convincing and persuasive which helps alleviate the issue of reasonable doubt in major cases. The experts background and credentials will also be a factor considered by the jurors.