I believe I already mentioned a plethora of reasons why it would be important to implement Day/Night:

To summarize:

Suggesting replace Long Rest button with a single button that transitions from Day to Night. Call it Long Rest button instead of End Day.

Reasons:
1. Ambience. Night travel in an RPG is fun for a lot of people. It creates a whole different atmosphere.

2. Long Rest 2x per day, so it feels more like my characters are doing more per day than what is in the current game. As it is, my characters wake up, adventure 5-10 minutes, and someone says they need to call it a day. It's been a long day. Adventure a little, rest a lot. That's BG3 right now. The grove ritual lasts for days upon days and never completes. Wyll never gets upset about not going after Spike even after a week - same with Lae'zel and the Gith, etc. Half the number of days that pass because you can do 2 Long Rests per day, and suddenly it makes more sense that the ritual isn't complete yet, Lae'zel hasn't left the party, and same with Wyll.

3. Drow and other creatures are sun sensitive. It's kinda a big deal. They could and should actually implement sun sensitivity since they even list it on various characters in the game. But IF they implement it, and then force Drow, etc. to take disadvantage because they have to roam in the sun, that's dumb. They should only implement it IF they give us Day/Night so Drow can actually stay out of the sun like they're supposed to.

4. Drow have superior dark vision, but many races have dark vision. This gives them an advantage over humans and other non-dark vision races. The Gith patrol, for example, would be much easier to win - and a new, more intelligent strategy - if you fought them at night. Astarion and others could remain in the shadows. Thus, enemies would get disadvantage on attack rolls against them if they remained outside the light. That could be an incredible strategy to turn the tide of that very tough fight on the surface. This alone turns the game on its head and adds a whole new level of strategy.

5. Advantages on Stealth checks while moving about at night. This is good for certain quests like sneaking into the goblin camp or through Moonhaven or sneaking up on the Hag's place or the Swamp Docks, or sneaking up on the Gnolls or the Gith or pretty much sneaking up on anyone on the surface.

6. Gives players the ability to make time pass within a single day. Thus, players actually have more control over the passage of time. How could this come into play? You see the burning Waukeen's Rest Inn. You play through the scenario. You Long Rest. It's night. The inn is no longer burning. Still the same day, but time has passed. A bit more immersion.