Originally Posted by Fat Mr.lol
It lets you hit Tyranitar without Aura Sphere, meaning you can now use Fire Blast without worrying about Tyranitar walling you.
+1 252 SpA Life Orb Mewtwo Psystrike vs. 4 HP / 0 Def Garchomp: 376-445 (105.02 - 124.3%) -- guaranteed OHKO
+1 252 SpA Life Orb Mewtwo Psystrike vs. 0 HP / 0 Def Rayquaza: 395-465 (112.53 - 132.47%) -- guaranteed OHKO
+1 252 SpA Life Orb Mewtwo Psystrike vs. 248 HP / 8 Def Giratina: 308-363 (61.23 - 72.16%) -- guaranteed 2HKO
+1 252 SpA Life Orb Mewtwo Ice Beam vs. 248 HP / 252+ SpD Giratina: 291-343 (57.85 - 68.19%) -- guaranteed 2HKO (against Giratina-a, Ice Beam will only do more damage against the physically Defensive sets, it will not KO though).
I mentioned Garchomp and Rayquazza because there will be times where Mewtwo KO something without a Calm mind under its belt, and a full health Scarf Garchomp or a Ray packing Extreme speed comes in, Mewtwo can survive a hit, and that is when you want to OHKO them with ice beam, and Psystrike fail to achieve this.
Anyway, I do agree GK has its merit, but ice beam is also useful, so I don't know.