Several questions are likely to arise in a commissioner's mind. The expert's role is to provide this information to the Court. The expert's role is to convince. To be effective, experts must be convinced of the correctness of their evidence. Conviction shines through expert evidence and imbues it with credibility.
Conviction helps make the evidence shorter, clearer and more relevant.
The expert's body language communicates it loudly and clearly. Experts who are ill at ease with their opinions tend to qualify their answers, sometimes to the point of losing track themselves of what they are saying. Most expert reports are longer than they need to be. I should add that this applies mainly to experts who argue against a proposal than to those defending it, since the latter are required to cover all possible angles of attack. They cannot tell which issue will eventually turn out to be significant during a hearing.