I don't remember day and night doing a whole lot in BG1, but it sure did a few things in BG2. Some merchants disappeared at various times, some appeared, a number of quests had requirements related to the time, and another number of quests literally had "do this within x hours or quest fails" conditions.
And don't forget the shadow temple where it had quite the impact that the game automatically switched to night time, playing directly into the shadow mechanic in the exterior.
That was frankly quite nice, even if it wasn't the most complex implementation. And it sure blows BG3's "everybody is always standing in the same spot" approach out of the water. The druid grove was sort of bad, with those tieflings always screaming at that apprentice guy. Or that other fellow who left his job after three days, in the middle of his shift. Now imagine how awful Baldur's Gate, the city, is going to be, with a swarm of NPCs that are all rooted to the same place and constantly and without any kind of interruption screaming the same thing, over and over and over and over and over, never even taking a potty break.
