So, why would sleep be benefitial to non-nocturnal animals? That is, I think the parent is saying that sleep must be very important because almost all animals sleep. If sleep wasn’t very important, a lot more animals wouldn’t sleep.
Given the path evolution takes it seems a stretch to ever consider it best, perfect, good or anything other than "it worked just enough at the time". Just enough being the key. All we know is that sleep didn't disrupt animals enough to entirely fail the evolutionary test. However plenty of garbage features go along with more powerful features. One in isolation is not proof of it's perfection, I would think.
Sleep wasn't a liability to our survival, especially our ancestors. It actually aided in our survival. Human beings who were active at night would have been the vulnerable ones as they would attract predators and were more likely to perish as a result. Especially considering out vision would be limited in the dark.
In short, sleep kept us away from the dangers at night.
Imagine a hypothetical state of pseudo-sleep that causes us to withdraw during the night (the same as normal sleep), except we keep our eyes open/ stay alert.
We are just as hidden from predators, but now even more equipped to defend ourselves.
Therefore it seems difficult to deny that sleep itself has some form of rejuvenative benefit.
Please try reading my comment or following the thread. I'll paste it here : "I was specifically addressing his point about the drawbacks of sleep - "There are several survival drawbacks from sleep, as it leaves one vulnerable."."
In other words, sleep has rejuvenative benefits and it keeps us away from predators. Sleep can have many benefits. You can stay up all night, but you'd miss out on the rejuvenative benefits of sleep. Or you can safely tuck away and sleep and gain benefits of safety and rejuvenation.