When there is a shock to the economy, social safety nets dampen the pain significantly. Local and state governments would never be able to provide a reliable social safety net, because they have to operate on the same principles as every other user of a currency. The federal government can operate on the principles of an issuer of the currency. That is a big difference.
In fact, you can see this difference play out in slow motion in the Eurozone, which still has not recovered from the financial crisis. Things would have played out very different had their been a Eurozone federal government that provides at least the basic components of the social safety nets.