Board Thread:Dark Souls II/@comment-86.142.12.210-20140318170858/@comment-24684538-20140318201911

Not entirely sure, I know Nashandra is the recollection of the scattered fragments of Manus. Maybe the Throne is where those fragments go to recollect or maybe it's there to watch over the Abyss to ensure Manus does not go out of control like last time.

I'm more inclined to believe the latter since the giants' torsos help you cross over to the Throne, since Nashandra convinced Vendrick to attack the giants I can't see why they would help Nashandra. Perhaps they are willing to help anyone who wants to prevent Nashandra, and therefore Manus, from causing more damage.