In a car, what is the difference between a 12-V and a 24-V system?
24V is used on trucks as the starter needs enough power for the big diesels : approximately 1000A. If that was at 12V then it's double the current which means thicker cables.

The disadvantage was that the bulbs needed longer filaments for 24V and the vibration (road and engine) caused them to fail often, this was helped by supported filament bulbs so that they lasted longer, now LEDs have helped even more.