Strictly speaking, Morpheus held up a battery at the end of his little talk. Humans make no sense as an energy source, obviously, but fat has a reasonably solid

Strictly speaking, Morpheus held up a battery at the end of his little talk. Humans make no sense as an energy source, obviously, but fat has a reasonably solid energy density for its volatility. My personal headcanon is that the machines were laboring under something akin to Asimov's three laws, which required that they preserve humanity. The Matrix, then is a loophole, something which technically satisfies the requirements of their core programming, but in a way that is advantageous to the machines. But then, once you've got all those people hooked up, you might as well make some use out of them. They aren't the most efficient battery around, but if you've got to keep them anyway, they might as well serve as a load balancer. The people of Zion just came later and mistook an ancillary benefit for the machine's primary purpose.