In the white paper it mentions that this provides implicit margin trading. If you suspect the price of eth is going up, buy eth, generate dai in a CDP. Use it to buy more eth. Use that eth to generate more dai. Rinse and repeat as the price of eth climbs. Then when you’re ready to cash out, use the remaining eth to buy dai “cheaply” and cascade unwind all the CDPs. In the end, you’ll have more eth that what you started with (a lot more) AND the eth price will be higher.