From the World Heritage inscription:
Hoi An Ancient Town is an exceptionally well-preserved example of a South-East Asian trading port dating from the 15th to the 19th century. Its buildings and its street plan reflect the influences, both indigenous and foreign, that have combined to produce this unique heritage site.
Hoi An is one of the stops on the typical tourist route from Saigon to Hanoi. A very small town near the sea, it was a trading port for hundreds of years. It has an architectural style unlike what you will find in the rest of Vietnam.
It isn’t the flashiest of sites, but it is a nice city and worth while place to visit if you are in central Vietnam.