Or as written by @steemrollin in his great post Steem: Where DOES the money come from?
"Steem
Steem is similar to publicly traded company shares. The amount of Steem doubles each year so there is signficant dilution for those that hold Steem, however like many fast growing startups a rapid increase in value may outpace the dilution. Just like company shares (ie. stocks) these digital assets have value as more people use and contribute to the ecosystem. The estimated value of the total outstanding Steem assets is currently about $20 million on Coinmarketcap.com."