http://money.cnn.com/2017/09/13/technology/social-security-number-identification/index.html said:
Social Security numbers were first issued in 1936 -- "for the sole purpose" of tracking the earnings history of workers for benefits, according to the Social Security Administration.
Until 1972, the bottom of the card said: "FOR SOCIAL SECURITY PURPOSES -- NOT FOR IDENTIFICATION."
But Social Security numbers have since become a key method of ID.
"It was the one unique piece of information that would identify every consumer individually," Neal O'Farrell, executive director of the Identity Theft Council, told CNN Tech.