"It has such good stats"
Yeah. I agree with star. That's kind of it :P and it's access to moves such as Will-o-wisp and Toxic.
Mew's all rounded base 100 stats mean it can function in many roles, and it's ability to learn any TM out there compliments that. Basically, it's like Celebi... except with a worse typing.
Anyway even in UU, there are stronger sweepers than Mew, but it's balanced defenses allow it to function as a wall much better than others (And access to instant recovery)