If you think of the value of Steem as the money supply per day, i.e the amount of Steem Dollars printed each day then you'll see they both go hand in hand. In order for an economic system to function it needs a currency in enough circulation to exchange goods and services.

The speculators are buying because of the above point, the money supply and the value of the network.

As for the demand for the Steem Dollar, the demand that generates a need for an ever increasing money supply - well that comes down to the sidechain and subchain 'industries' that can be built. I'll go into more depth in the next part.

Thanks for your feedback.