Another way to think about opening and closing CDPs is that this is essentially going margin long on the collateral asset in a decentralized way. For example, if a user believed that ETH will appreciate in value over the next few days, s/he could open a CDP for 500 DAI collateralized with 2 ETH at $500 per ETH (so $1000 worth of ETH). The user then immediately takes the 500 DAI to buy one more ETH. The user now has 1 ETH that they own and 2 ETH locked in their CDP. If the price of ETH rose to $1000 per ETH, then the user only needs to sell 0.5 ETH for $500 to reclaim their CDP, leaving the user with 2.5 ETH, which is not only 0.5 more ETH than they originally had (2 ETH), each ETH is worth substantially more than before.