From the World Heritage inscription:
The island of Itsukushima, in the Seto inland sea, has been a holy place of Shintoism since the earliest times. The first shrine buildings here were probably erected in the 6th century. The present shrine dates from the 12th century and the harmoniously arranged buildings reveal great artistic and technical skill. The shrine plays on the contrasts in colour and form between mountains and sea and illustrates the Japanese concept of scenic beauty, which combines nature and human creativity.
Miyajima is a great place to visit. A tram ride in Hiroshima to the dock and a short 10 min ferry ride can take you to Miyajima. In addition to the Itsukushima Shrine (Shinto), there is the Ottori gate (one of the three great views in Japan), the Daisho-in Temple (Buddhist), and many stores and small restaurants, and tons of small red deer all over the island.
I found Miyajima to be the high point of my visit to Hiroshima.
View my complete list of UNESCO World Heritage sites.