But the real question is why this particular banking service exists? If Bitcoin is the stable currency it was designed to be, then it would be nonsensical for a person already holding it to borrow 30% of its value in USD at 8% APR. The answer of course is that some people are so convinced that BTC will rise in value by more than 8% over the loan duration they're prepared to go into debt to keep large portions of their net worth in it, and some might even fancy buying more BTC with the cash they've borrowed, and maybe borrowing against that too. In this analogy BTC isn't the bank, it's the houses people were taking out the subprime mortgages on (minus the upside of avoiding rental payments)