I could be wrong, i wanna see it again, but I think Zion is created to prevent the One from destroying the matrix.
The One occurs because humans are given choice.....
He is the anomoly that could threaten the existance of the matrix, therefore the computers/ machines need some way to control him.
Everytime The One has the choice to destroy the matrix he has to choose between Zion & Trinity or choose to destroy the matrix. Everytime he chooses love, he loses, Zion is destroyed and the one is brought back to choose the new males and females to rebuild Zion for when the next anomoly occurs.
I think.....
But if the computers needed a power source why did they not just use cows instead, they're bigger providing more power and they're not likely to rebel. Also you could call it the MOOOOOOTrix
Heaven doesn't want me, and hell is afraid I'll take over!