At some point it's considered that we might use them as currency between us. Also, given the amount of people wanting to cash out of Steem Dollars today - you might be able to buy them at a significant discount. Then once the markets get deeper you could realize that gain.
I guess their purpose is to solidify the USD value of rewards. If the Steem price is illiquid and flip-flopping everywhere, the Steem Dollar is a way of keeping that value fairly solid.
That's the best answer I know. If @dan or @ned want to give a better answer, they're welcome