Intimidation.
Gyarados comes onto the field. Say, for lack of a better scenario, you have a fire pokemon, Charizard for instance. Now, you have two choices ahead of you.
Either A) Switch to a counter, who can drive the beast back, expecting a Super Effective attack.
or, B) Stay in, predicting the DD.
Now, the results.
A=Gyarados DD's on the switch, and proceeds to damage your Counter, who throws down with the beast and effectively Charge Beam/Thunderbolts it.
B=You've stayed in, only to give your opponent minimal damage to the bulky gyarados with a NVE move, or a neutral move. The only thing you gain by this, is small damage which gyarados may survive another/2 of, giving your opponent ANOTHER opportunity to DD. Then, it will DD a second time. Here, your counter is now up against a Speedy, STAB Waterfall off of 780ish attack, Let alone the possibiliy of Ice Fang/Earthquake, and almost assuredly Taunt. So, no phazers. The only thing that may help you in this case is Cresselia, who has the possibility of KO'ing with Charge Beam, but may not take the abuse of STAB +2/+2 Gyarados so well. Even so, they've wrecked your wall, and possibly another pokemon with it.
It's always better to switch, unless you're assured a KO by staying in. However, Intimidate lessens the chance of things like Charizard and Infernape from hitting too hard with Stone Edge after the attack drop.
Put them in a position where they risk losing a pokemon, and almost always, they'll choose to err on the side of caution.