I saw a special about this on the discovery channel... I think they said it was to control floods, although it will be used for power, too. Floods cost the government much $$ in China.As Michael said, it will force many people to re-locate, mostly peasants who have lived there for generations, who are used to living as if their entire village was part of their family. It will also destroy many religious and historic buildings and landmarks of much importance.
There are many dangers, too, such as silt build up and what could happen if a flood hits while they are building the dam. The chinese think it will work, but there are man experts, including a leading US engineer that think the project will end tragically.
*marcc*