Comfort and Convenience
Cabin temperature is the clearest example: what one person finds pleasant, another finds stuffy or chilly. Build, clothing and where someone sits relative to the sun all play a part, so a quick word before setting off helps.
Safety and Driver Assistance
Setting mirrors, seat position and lights before moving off is simpler while stationary than in traffic. A few minutes spent locating the essentials in an unfamiliar car means fewer interruptions later, when passengers are talking or the route is new.
Multimedia and Connectivity
Battery level is the usual worry on longer trips, since phones are often handling navigation and messaging at once. If a car offers a charging point, agreeing who uses it avoids the small negotiations that crop up mid-journey.