It counters rain teams. Not just grass, but whole rain teams. It uses STAB Rain-Boosted Hurricane to plow through Pokemon that think they can wall this beast, and HP Water (also boosted in rain) to hit Ground Types like Landorus-T. One Quiver Dance can do some work. Here are some damage calcs against common OU Pokemon (this is with a +1 Quiver Dance Boost, 252 SpAtk EVs and a Special Attack increasing nature):
Hurricane:
vs. 4 HP / 0 SpD Talonflame: 275-324 (92.2 - 108.7%) -- 56.3% chance to OHKO
vs. 252 HP / 4 SpD Landorus-T: 243-286 (63.6 - 74.8%) -- guaranteed 2HKO w/ Leftovers
vs. 4 HP / 0 SpD Gengar: 257-303 (98 - 115.6%) -- 87.5% chance to OHKO
vs. 252 HP / 4 SpD Azumarill: 243-286 (60.1 - 70.7%) -- guaranteed 2HKO
Hidden Power Water:
vs. 0 HP / 4 SpD Terrakion in Rain: 362-426 (112 - 131.8%) -- guaranteed OHKO
vs. 252 HP / 4 SpD Aegislash-Blade in Rain: 378-446 (116.6 - 137.6%) -- guaranteed OHKO
vs. 252 HP / 4 SpD Mega Tyranitar in Rain: 284-336 (70.2 - 83.1%) -- guaranteed 2HKO
Yeah. Crazysane. This set is explained more here.
OU usage stats from here.
