Mostly as an anti-landing move. If an opponent tries to come down right on top of me, then U-Smash typically puts a stop to it. Even if the opponent manages to air-dodge through the bulk of the move, they still tend to get sucked into it for the last few blows unless they're playing as a really short character, like Kirby or Jigglypuff. Also, Mewtwo's U-Smash can actually power through moves like Falcon Kick and Wizard's Foot, as well as a few other stall-then-fall moves. It's pretty incredible, and the opponent is in for a rude awakening if they don't know what they're up against.
In addition to that, I occasionally use it to counter opponents who try to jump from a ledge, and if anybody happens to be on a platform above me, then Mewtwo's U-Smash can usually hit them through the platform. Finally, I also sometimes use it as a follow-up to a D-Throw. It's a pretty effective kill move, as well, like you said. I don't know if it's Mewtwo's best kill move, but it's certainly up there. I really enjoy using it!