IF you mean PnP, it can destroy the will to fight of the enemy. If you make them a laughing stock enough. It also helps your reputation, meaning if you consistently declare and win, declare and win, declare and win then you get more respect. Also, like previously said, if you have that track record and then declare on someone you destroy their will to fight. And in wars, that is what matters. Not villages taken.
However, it is a double-edged sword. If you do it wrong......