The reason for going outside the Empty is...
It was explained that in order to summon the correct aspected primal they had to be in the right region with the appropriate element, i.e. a rocky/mountainous region to summon Titan.
Then, hopefully, after the primal was defeated, its aether will go back into the land and rejuvenate it. So, the mountains and the land around Titan changed back to brown and normal looking after he was slain. It will then spread from there to other parts of the Empty.
If you remember, after we defeated Leviathan, only the immediate area around Eden was affected.