Lower interest rate .... which helps keep mortgages at a lower cost to the consumer. Frankly this IMO is more of a competitive advantage in competition with private label MBS developers. The cost of doing business for F and F was lower.
Liquidity in the after market where some might say 95% of the action takes place (as with most bonds).
The need for liquidity (ease of buy and sell at good prices not inflated by "fear") would not be met with a lower interest rate or direct subsidy to F and F.
Only "knowledge" that there is an ultimate backer of the paper does this job
While some can argue - the "implicit" guarantee (or market assumption of a guarantee) did this job
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