When people ask me where the value comes from I always say this.
The reward pool is like printed money (currently about 10% a year) and if you don't participate in the platform your value is decreasing and that is how an author receives money.
Also the entire value increases as more people invest with the speculative idea that steem will grow bigger. Then just like most platforms after a few years of only increasing in value due to speculation it will start to generate more of it's value via subscriptions or advertisements.
In steemit's case a subscription might look like someone buying steem but not participating enough to keep ahead of the inflation, and therefore need to continuously reburying steem.
and the advertisement might look like a company/person buying steem so it can promote it's own content.
Anyway thats my take on where the value comes from, do you agree or am I way off the ball. @ned