Not sure how night impacts gameplay in D&D though. Apart from pure roleplay and infra-vision elf friends is there anything that changes for the way you play your character? Talking mechanics here, not roleplay wise obviously.
- Some creatures (i.e. vampires, for one) would be able to be around just at night. Or just be more active/effective then. - It would mark the passing of time, making the introduction of tiredness/exhaustion somethings that makes sense in context. - Consequently, it would mark the necessity to rest at least every now that then.
And that's obviously without going into all the large amount of possibilities it opens in terms of scripting and quest design (which on the other hand would be something Larian would need to purposefully take advantage of) and the already mentioned benefits in terms of cosmetic value/immersion.