It says that it was origionally, because of it's size(It was the highest mountain on cocytus) and because it had five spires(The Cocytans had a special reverance for the number five, it says in the same section), called the Sacred Mountain, and henceforth they hollowed out the bowl shaped depression, the spires, and filled the spires with all the wonders of cocytan technology, all of the things they considered momentous or incredibly special including the tomb of the creator. They also placed samples of every animal they could find, the more dangerous confined to force fields which broke down a long time after the cocytans went into spacetime six. This is all taken, again, from the strategy guide. I need to type up the whole section really and put it on my site.
As for why the cocytans would need humans again, perhaps we don't know they need them at first. The Cocytans invite the origional crew and some of earth's finest minds and some politicians to take a tour of the changed Cocytus. While there, they discover some kind of sinister secret or conspiracy against the Cocytans, and in the process one of the politicians is killed, the death being blamed on a prominent Cocytan. Now the origional crew and the newbies have to help the cocytans yet again while saving the relationship between Cocytus and Earth.
Edit-I forgot to put something.