I would say the most likely answer is starting a new job as the social security number is used to track lifetime earnings and and number of years worked and is also used for taxation purposes. In Canada, it is used as ID and in part for eligibility for employment insurance, public pensions, education and training and disability support among other things. In the US it was first introduced in 1935 as part of FDR's New Deal program to help people during the Great Depressio