Basically, Kishimoto realized around the start of the kage summit that, oh crud, the titular character isn't as strong as Sasuke as the readers probably expect. So he gives him these massive fox powers to boost him up. Also, he's the prophesized child, the son of the fourth hokage and a clan of incredibly powerful seal users branched off from the senju clan, and has the ability to befriend all the pokemon ala pokemon ranger minus the spinning tops. So now Kishi realizes, oh, now Sasuke's just your average Uchiha. I need something to give them both equal ground.
And thus the quickly created and not well thought out creation mythos was born. Also Sasuke's a good guy now
All hail Naruto! The ultimate hero of all heroes!