Comfort and Convenience
Cabin temperature is the classic point of disagreement: what feels pleasant to one person can feel stuffy or chilly to another, partly because seating position, clothing and airflow differ from one seat to the next.
Safety and Driver Assistance
Sorting out wipers, lights, mirrors and demisting before setting off is simply easier than doing it in traffic with passengers aboard. A few minutes stationary means fewer things to work out later.
Multimedia and Connectivity
Phone battery is the usual worry on longer trips, since navigation drains it quickly. Where a charging point is available and reachable from the passenger seat, someone else can manage it without the driver reaching across.