Have you ever spoken to someone who couldn’t appear to be able to stand still? That can get annoying pretty quick can’t it? When you speak with someone, standing still communicates power and confidence in what you are staying, even if there isn’t a shred of certainty in your mind.
I am not against making minimal and appropriate hand gestures as one speaks, but I am against aimless movements that distract and take away from an otherwise better interaction. Unless your movements have purpose, stand still as you speak. It’s better for everyone.