The United States advocated this plan as a way to contain communism.
After WWII, the economies of Western Europe were devastated. The bombing and other fighting had destroyed much of the physical infrastructure of the region as well as killing and wounding millions. This left the countries of the region in perilous economic condition.
The United States did not think this was good for the world. Most importantly, they felt that countries that were economically weak and socially devastated would be fertile ground for communist insurgencies. They felt that helping the economies to recover would make the people of the countries more content with democracy and capitalism and less likely to become communist.
The Marshall Plan formulated under the Truman Doctrine aimed to provide economic assistance to European nations that were in the midst of post-war reconstruction. The Americans also sought to use the plan as an attempt to contain the spread of Soviet influence on the European continent, especially in Eastern Europe. This was achieved through the establishment of economic ties with the US, which in effect would break any bonds these states had with the USSR. Policy-makers in the US also sought to use the economic recovery of European nations as an outlet for the excess industrial output the American economy was generating.