Curse is used to not lose to certain threats like Coil Zygarde without using gimmick HP Ice which is risky with Weakness Policy being used. It also helps vs :What's generally better, SD or Curse Scizor?
DD Mega Gyarados without Taunt
SD Mega Mawile without Fire Fang (nobody uses SD+Fire Fang)
SD Mega Scizor: you can also Curse once and slow U-turn out into something like Specs Lele
Mega Pinsir (it has to be a 1vs1 situation)
SD Kartana ((it has to be a 1vs1 situation if Z-Sacred Sword)
If you often lose to these mons and you rarely sweep with Swords Dance, Curse might be the better option.