Comfort and Convenience
Cabin temperature is a common point of difference: one person settles happily in cool air while another prefers warmth, and those preferences shift with clothing, seating position and how long the journey has already run.
Safety and Driver Assistance
A few minutes before setting off, spent locating lights, wipers, mirrors and the hazard switch, leaves fewer unknowns later. Preparation is simplest while stationary, with no traffic or passenger conversation to work around.
Multimedia and Connectivity
Keeping a phone charged over a long trip is a separate question from playing audio through it. Where onboard power is available, a passenger can handle the cable and the handset so the driver need not.