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Basically you are borrowing bitUSD with an interest rate of 0%. That's why it's so attractive.

If the bts exchange rate falls though, then you might be forced to sell at a bad rate, so always check that you have locked enough bts in collateral.

The interesting part is that the bitUSD is created "out of thin air" when you borrow them and disappear when you pay them back.

And that's why people think more and more bts will be taken out of circulation, because it's so attractive to put it up as collateral.

so you keep your bts, and borrow money for free at the same time if you're not too risky.