Day and night cycles could certainly serve a purpose, but I am not entirely sure the story is adapted towards that? I mean, some "missions" would ideally take place in the evening or night, like stealth quests in the city or something, but other than that I guess it's mostly an atmospheric thing?
I mean, until the feature is there you can't even begin to tailor content specifically around it, but the possibilities it open aren't exactly negligible.
There are entire categories of monsters that are supposed to roam mostly or at times even exclusively during night or daytime, for a start.
And that's without even going into urban encounters with NPCs of all sorts on the streets of the city.
