[catbird]

I think the line you're referring to actually read "For social security and tax purposes - not for identification." As it was explained to me, it meant that the card itself was not a proof of identity, as opposed to something like a drivers license that had a picture and other identifying info.

https://en.wikipedia.org/wiki/Identity_documents_in_the_Unit...

[Nomentatus]

In many countries it was originally illegal to ask for an SSN(equivalent) as identification; the line suggests that was originally true in the U.S. as well.