To answer your first question: the benefit of derivatives, of which CDOs are a type, are that they theoretically allow people to _reduce_ risk. The classic exa

To answer your first question: the benefit of derivatives, of which CDOs are a type, are that they theoretically allow people to _reduce_ risk. The classic example is a put option. If you think a stock will go down in a future, you could short that stock; but if instead the stock goes _up_, you could be on the hook for an infinite amount of money. Instead, you can buy a put option and get a similar payout if the stock goes down, but if it goes up, you've only lost the money you spent on the option. Another example is purchasing futures: if you're an airline and you think fuel prices are going to go up, you can buy up a messload of fuel futures, and you'll effectively pay the same price for fuel while everyone else is paying more (which I think JetBlue did in the 00s). In this case, a CDO theoretically reduces risk by spreading your risk across a huge number of mortgage holders. If you sell someone one mortgage and they default, then whoops! You just lost a ton of money. But if you instead sell 100 different mortgages, you're "diversifying" your risk. Most people don't have billions of dollars to sell thousands of mortgages to make this sort of investment, so CDOs package the mortgages into purchaseable portions. (Of course, what happened during the housing crisis was that there was a huge, systemic issue that would cause huge swatches of mortgages to default. That part sucked, and the contributors to that catastrophe definitely deserve the ire on them.) [Disclaimer: I'm relying on my memory of finance classes I took 15 years ago, I'm probably wrong about some details.]