The purpose you say. Well, since the dawn of time. There has been 3 Legendary creatures in each of the Regions. The 3 Birds of Kanto, the 3 Beasts of Johto and the 3 Golems of Hoenn. The purpose of them is actually unknown to most, but they are there to test your knowlegde of Braille. That all really, but it isn't. There just there because you they needed 3 more legends to put in the game. The only above average stats of them are there Defenses. Huge, like walls. I have all 3 and they haven't been out of thier box since. For me they are only for Pokédex purposes only.
Master Mewtwo