Why is the Audi A2 known as a pioneer in lightweight construction?

I've heard the Audi A2 is considered a pioneer in lightweight construction but I don't understand why. Can someone explain this to me?

The Audi A2 was the first mass-produced car to utilize aluminum in its construction, making it significantly lighter and more fuel efficient than its competitors at the time. In fact, it was about 40% lighter than other cars in its class. Aluminum also has the advantage of being rust-free and more easily recyclable, making it a more sustainable choice for car manufacturers.