There are two answers to that.
The one assumes he has any control over what he says.
Assuming he doesn't, well, the gain question is not a question. It isn't about gain. It is about an out of control narcissist who seriously thinks that nobody credible knows how to hit rewind and consider his contradictions. So of course anyone who does is absolutely a liar.
If he has any control, that speaks even larger volumes about what he thinks about the intelligence of the public. It means he thinks he is ahead if he feeds his crowd with what they want to hear and that his utter lack of credibility in the face of the flimsiest of evidence has no consequences.
Which is scarier? The person who is doing this irrationally or rationally?
Manufacturing consent is one thing. This may be uglier. If you look at the historical (and even contemporary) cases of this it got much much worse.