"... but why at all did SS become needed???"
It really didn't become needed. FDR and the Congress created it for political reasons, just like so much of the "entitlements" that have since been created. It's all about politics and votes. Where is the Constitutional basis for all of the federal "entitlements"? It does not exists, but the programs have been allowed to exist despite the lack of a Constitutional basis. Laws and the constitution only are effective if enforced in the courts.