There are three currencies, Steem Power, Steam and Steem Dollars. Think of Steem power as vesting shares in a start-up, steam is the shares you trade day to day and the steem dollar if your paycheck. The reason for the Steem Dollor is because everyone knows what a dollow is and what it is worth( capable of buying). As Steemit adds features like a marketplace or craigslist, people will be more comfortable spending in a form they are familiar with. If I say something cost 50 dollars you know what that means, if I say it cost 24 steem, you have to first convert that to something you know and then hope steem's price stays the same until you make the purchase. Too much effort for people to need to do all that.