[lazide]

SSN is used by many, many entities as a 'secret security code' essentially. Including (still) many banks, government offices, etc.

It's also used for things like tracking of tax liabilities - so if someone has your SSN, with some minor fudging of other data there have been historic issues like claiming tax overpayments and getting checks from the government.

It's incredibly dumb, but it is what it is (mostly still).

At least most of the big players aren't quite as dumb about it as they used to be.