What purpose does that serve? Why would it be more advantageous than a continuous time stream?

We don't have similar separation between morning & evening, for example. (though I've always been a fan of the siesta)

Its meant to serve physiological homeostasis in some way yet to be completely understood :)

as someone who has had a rotating schedule their entire life, I wonder what that would mean for my brain.

What do you mean by your "entire life"? Since you were an infant? I have non-24[1], but it didn't manifest until I was in my 20s.

[1] https://en.wikipedia.org/wiki/Non-24-hour_sleep-wake_disorde...

meant by who?

Whoever you want.