It's good for:
1) Targets weak to Psychic
2) Targets weak to Ghost
3) Targets whose viable counters are relatively low-damage, like Electric
Because Mewtwo has 3 roles having a powered up one can allow it to fit into a variety of teams that might not be all the way filled.
Also has an anti-Blissey role with Focus Blast, but I haven't tried this yet.