When social security was created, people were fearful of a govt that would use it for all kinds of nefarious reasons, and use it as an all purposes tracking number. The people were promised it was for one use and one use only - to make sure the banks could verify that no SS fraud was taking place, and to safely deposit their funds at collection age.
Now it's used exactly how we were told it wouldn't be. My understanding is that this is technically against federal law, but I have no sauce for that, I'm going by old memory. picrel.