My understanding of the push for a 48V automotive power scheme was that it opened up the possibility of operating the intake and exhaust valves with solenoids, allowing for computer controlled variable valve timing systems, and eliminating the frictional losses associated with the timing chain/belt, camshaft, lifters and other valvetrain components. At 48V, the current draw of the powerful solenoids that would be needed would be low enough to be easily switchable with reasonably priced power semiconductors.
If cars went to a 48V power system, accessories like A/C compressors could be driven electrically at a constant speed, enhancing efficiency, and relocated out of the engine compartment, enhancing reliability. Power steering systems could eliminate the hydraulic system, and use an electrical assist unit.
There would be no reason that a simple DC-DC converter couldn't be used to provide 12V for things like stereo systems or the lighter socket that 12V equipment gets plugged into.
