There were two important and interconnected main effects of the Marshall Plan. First, the Marshall Plan helped the economies of Western Europe recover from the war. Second, the plan helped cement the friendship between the US and those countries and helped to keep the European countries from moving closer to the Soviet Union.
After the war, the economies of Europe were devastated. The US feared that extreme poverty in these countries would push them towards communism. Therefore, the US created the Marshall Plan in order to kill two birds with one stone, so to speak. The Marshall Plan simultaneously helped the countries of Western Europe to recover from the war and made them more likely to ally with the US against the Soviety Union out of gratitude for the aid.