Virtually all electronics need a step down (buck) converters as they run at lower voltages 5, 3.3, 1.8. 12V > 3.3- 1V would a single step. 48V ones would likely require an intermediate step. The only exception would be running some power systems where it'd require less current.
The main savings is current though, because the wiring harness is one of the most expensive parts of a car.
There are a few different topologies for a 48v harness, but somewhere in the line there's a 12V DC/DC converter in there somewhere.
The wiring for 48V can be a lot thinner than it is for 12V. As there is a square law involved for resistive heating it turns out that wiring for 48V can use 1/16th of the weight of copper as that for 12V.
A switchmode converter can be designed for 48V just as easily as 12V.
You generally can't reuse non-automotive power supplies in automotive because the requirements are very different.
Hobbyist computing could benefit from a move to 48V as well, if only to keep the problematic 12VHPWR from killing expensive video cards.
I say hobbyist because AIUI 48v is making inroads in server hardware but that's not my area.