It's just another underground forum, the purpose of which is the distribution of illegal goods. There are many others like it. It's only special because it uses bitcoin as the means of payment. So it is much more advanced in that regard.

They also have a hedging feature, so if you want to sell something, and don't want the hassle of shifts in the value of btc:usd exchange, you can hedge your account in USD so if btc price rises, you have less btc but same USD, and likewise, if price reduces, you have more btc but same USD value.

The only reason it is important in providing an economic backdrop for bitcoin is that the majority of legal trade websites are not using bitcoin yet. Once it does, SR's "value" to the bitcoin economy will be much less significant.