If you look closely in the whitepaper you will notice that just for holding Steem Dollars you can get 10% interest. This creates a huge market for Steem Dollars for investors. It might go on to explain why Steem Dollars are going for a price higher than the regular dollar because Steem Dollars are better than regular dollars.
Regular dollars do not provide any interest. In fact if you hold the USD then you are going to have to deal with inflation. Additionally if you hold Steem Power you have possibly 5% dilution if you aren't active. Steem Dollars on the other hand in the current economic circumstances are great because you can't get a risk free 10% interest almost anywhere else. You won't get that in stocks unless you really really know what you're doing and even then it could have an off year.
If Steem Dollars are used a lot on the Steem Platform then the growth of Steemit itself would be what backs the Steem Dollar. What can you do with this interest on your Steem Dollars? You can compound it by buying Steem Power.
I found this handy Steem Dollar Compound Interest Calculator for anyone who wants to calculate how much compound interest they can get.