48 volt is what the industry calls mild hybrid. It allows for much better stop-start systems, which automakers get points for on EPA and European emissions testing.

It also allows for 48 volt super-charging and e-assist turbo, as well as a small amount of actual propulsion power (for instance to both start the engine and start the vehicle moving from a complete stop after the stop/start system has been engaged).

All of these systems fall under the term "mild hybrid".