Why does the Audi A7 have 48-volt mild hybrid technology?

I've heard that the Audi A7 has 48-volt mild hybrid technology, but I don't understand what it is or why it's important. Can someone explain?

The Audi A7 uses a 48-volt mild hybrid system, which teams up a belt starter-generator (BSG) and a lithium-ion battery pack to improve fuel efficiency and reduce emissions. The BSG provides an additional 48 horsepower for short-term power boosts while also allowing the engine to switch off and coast at speeds between 34 and 99 mph for further fuel savings.