What is the purpose of the Audi A5's start-stop system?

Can someone explain to me why my Audi A5's engine shuts off at traffic lights with the start-stop system? Is it for increased fuel efficiency or something else?

The start-stop system was designed to reduce fuel consumption and emissions by shutting off the engine when the vehicle is stopped or idling for a long period of time.

I use my Audi A5 in daily traffic and I can tell you that the start-stop system does a great job of saving fuel and reducing carbon emissions. You'll notice a difference in your gas mileage over time.