In the dark, evil. EVIL ALL AROUND YOU IN THE DARK. How do you prevent evil? Light. Energy! GENERATORS!!!! We can't explain any better than that. Please, can we have a tissue? We're overcome with emotion after that performance.
In olden times, power had to come from fireflies or lemons, or perhaps a toad on a treadmill. Today, the same thing can be accomplished with generators. Also, it can be accomplished much better. Toads on treadmills aren't going to power anything. Why do you think they called it the Dark Ages?