The Economist has a new article out, End of Illusions. It goes over the history of Freddie Mac/Fannie Mae, how they operate and is implying the US financial system is a house of cards. (or in this case, Turtles propping up the world analogy, sorry Hindus!)
They (Freddie Mac, Fannie Mae) were set up (see article) to provide liquidity for the housing market by buying mortgages from the banks. They repackaged these loans and used them as collateral for bonds called mortgage-backed securities; they guaranteed buyers of those securities against default.
This model was based on the ability of investors to see through one illusion and boosted by their willingness to believe in another. The illusion that investors saw through was the official line that debt issued by Fannie and Freddie was not backed by the government. No one believed this. Investors felt that the government would not let Fannie and Freddie fail; they have just been proved right.
The belief in the implicit government guarantee allowed the pair to borrow cheaply. This made their model work. They could earn more on the mortgages they bought than they paid to raise money in the markets. Had Fannie and Freddie been hedge funds, this strategy would have been known as a “carry trade”
Investors have got quite a bit of protection against a housing bust because of the type of deals that Fannie and Freddie guaranteed.