The major purpose behind the establishment of the Social Security Act in 1935 in the U.S. was to provide a safety net for the elderly, unemployed, and other vulnerable populations during the Great Depression. It aimed to reduce poverty and ensure financial stability through a system of old-age pensions, unemployment insurance, and assistance for dependent children and the disabled. This landmark legislation marked a significant shift in the role of the federal government in the economy and society, emphasizing the need for social welfare and responsibility in addressing economic disparity.