Quash is generally a move for use in Doubles and Triples. It can be useful for revenge killing an opponent on low health by making them move last and having an ally finish them off without being harmed or guaranteeing a Pokemon gets to set up without worrying about a Taunt, for example.
There are definitely moves that were made for use only in Doubles and Triples like Helping Hand, and Quash is just another one of those.