Trump is doing this for the simple reason of brutalizing Clinton and showing that he can do so. Whether it makes any sense as a literal argument is really beside the point. It is at the root of the "bitch slap" mentality that power is demonstrated by inflicting harm on others and showing they can't fight back.
Among the responses from Marshall's readers, one wrote:
... Trump’s attempt to show dominance is directly related to his own feelings of inadequacy and ineptness. He’s been allowed to bully and intimidate women for decades, and when called out or pressed on it, he sues, threatens or demeans.