Some might think the actual atom bomb was built and assembled on-site at Hanford and then shipped off. That wasn't the case and here is the #1 contribution that the Hanford Nuclear site made during World War II:
The primary mission of the Hanford Site during World War II was to produce plutonium-239 for the "Fat Man" atomic bomb, which was dropped on Nagasaki, Japan, in August 1945. The production of plutonium required large-scale nuclear reactors to create the necessary conditions for fission, which would produce the desired material.
Once the plutonium was produced, it was chemically separated from the other reactor byproducts through various extraction and purification processes. The purified plutonium was then sent to the Los Alamos Laboratory in New Mexico, where it was used in the assembly of nuclear weapons.