From elcooksta's SS Link: "Generally, only noncitizens authorized to work in the United States by the Department of Homeland Security (DHS) can get a Social Security number. Social Security numbers are used to report a person’s wages to the government and to determine a person’s eligibility for Social Security benefits. You need a Social Security number to work, collect Social Security benefits, and receive some other government services."
Its amazing that non-citizens can be eligible for SSI benefits when it began as a "social insurance program" providing "economic security to U.S. citizens".