Why would someone join a battle with different weather or day/night?

In BG3, you start the game together as members of the same party. Though you could separate and go different ways, in order to join a fight, you'd have to move into the vicinity of your party mates. So as you approach the battle area, if it was raining somewhere else, it would stop raining as you approached the battle. If they built it so you can't transition from day to night without everyone agreeing to it, then you wouldn't have someone traveling by day and someone by night. Either everyone is traveling by day or night.

Maybe You're thinking realtime clock. I'm not. Day/night by short rest or something similar would make it so everyone must agree to transition between.