Processing power is my second favorite explanation.

My first favorite would have been: they don’t use the humans for anything, the pods are just the most efficient way to store humans. The machines think they are being benevolent, just want peace and quiet and for humans to stop doing dramatic things like scorching the sky. But I don’t know where the plot would go from there.

There is backstory that the films could have gone into, though I don't know if it was written before or after the first film. The humans in the matrix were allied with the machines and they put them in the matrix to protect them from the war. They were being benevolent.