It’s difficult for a fact-finder to judge the demeanor and weigh the credibility of a witness when her face is a piece of cloth. Such testimony would be a waste of time — I would probably give it little or no weight.
I’ve always found it troubling that judges pass judments based on things like demeanor and appearance.
I don’t see any good reason why they need to do as they do, but if you have one, let me know.