After I beat N and the game restarted, I was walking on the Marvelous Bridge when the Shadow Triad appeared around me. They gave me the Adamant Orb, the Griseous Orb, and the Lustrous Orb. In my bag, I saw that they were for Dialga, Giratina, and Palkia. But I can't figure out why I got them. Can someone help me out???
It's because items can't be traded over, and there's no other way to get the items unfortunately (which means the those three miss out on their special STAB-boosting item). Other than that, it's just to be nice really, which is a fun bonus to get at that point in the games!