From the World Heritage Inscription:
There are around 48 Buddhist monuments in the Horyu-ji area, in Nara Prefecture. Several date from the late 7th or early 8th century, making them some of the oldest surviving wooden buildings in the world. These masterpieces of wooden architecture are important not only for the history of art, since they illustrate the adaptation of Chinese Buddhist architecture and layout to Japanese culture, but also for the history of religion, since their construction coincided with the introduction of Buddhism to Japan from China by way of the Korean peninsula.
If you are in Kyoto and making a day trip to Nara, make sure to stop in Horyu-ji on the way. Horyu-ji is only a 20 min train ride from Nara, but is the often overlooked by tourists. The main temple is walking distance from the train station and the route to the temple has plenty of signs.
The wooden buildings in Horyu-ji are the oldest wooden buildings in the world. Achieving this feat is really more a matter of luck than anything else. Most wooden buildings over time fall to lightening or cooking fires. The main palace in the Forbidden City in Beijing has been rebuilt several times as have most of the significant buildings in Kyoto and Nara. The buildings of Horyu-ji are believed to also not be original buildings, but having survived from the 8th century, are still the oldest on Earth.
View my complete list of UNESCO World Heritage sites.