Click on the picture of Giratina.

Apparently, he has a new form in the movie called the "origin form." Notice how its height and weight are different from the Giratina we see in D/P. Maybe we'll see this form in the third game installment for the current generation?

That's really cool, even though I don't like this alternate form very much (the only thing that attracts me is the neck and head). It looks way more vicious (and ugly) then our ordinary Giratina, but if it's Giratina we're talking about, then it's probably awesome.

I wonder why and when does Giratina use that form, and for what purpose.

According to various Pokémon websites, including the home page (www.pokemon.com), Giratina's "Origin Forme" is used when traveling through dimensions. The Origin Forme will appear at Spear Pillar in Pokémon Platinum, as well as in Pokémon 11: Giratina and the Sky's Bouquet: Shaymin