This is theoretical.. One common answer is that it's a time to rebuild body cells. That's practical but not fundamental enough to form a true basis. I can contrast with the question of why we can't just have bodies that rebuild instantly with the food we input to our bodies; much like an engine (perpetually running for thousands of hours given fuel).I think the answer lies in the mind. Sleep's fundamental cause is to allow a "resolution" of mental input to resolve against an adjacent plane of existence (quantum physics if u will). Much like the human body needs a break from eating to do other business.So theoretically, if you have nothing to resolve, you can work forever if given enough food.
Imagine talking to someone who has shut off part of their brain, while still accepting stimuli. It's like dual-personality. "Hey Jeff, wasn't that kayaking cool? Jeff: What kayaking???"
OT: Also to complement, maybe some ppl work better with insomnia?