Why is the great barrier reef on the world heritage list

Why is the great barrier reef on the world heritage list as known as the unesco? Please give me as much detail as you can so I can understand it better.
Thanks so much for your time and whoever gives me more better detail, I'll give best answer.

"The Great Barrier Reef is a site of remarkable variety and beauty on the north-east coast of Australia. It the world's most extensive stretch of coral reef and is probably the richest area in terms of faunal diversity in the world. Its great diversity reflects the maturity of an ecosystem which has evolved over millions of years on the north-east continental shelf of Australia. The site contains a huge diversity of species including over 1,500 species of fish, about 360 species of hard coral, 5,000 species of mollusc, and more than 175 species of bird, plus a great diversity of sponges, anemones, marine worms and crustaceans, among others."

From the UNESCO World Heritage site
http://whc.unesco.org/en/list/154

it is a beautiful place with an amazing rich diversity of life that needs to be preserved for future generations