The Steem blockchain would go on working with or without us. Steemit is a business that runs one frontend interface for the blockchain. We use the revenue we generate to produce code that can be used by the Witnesses to improve Steem through Hardforks. Some of that revenue comes from selling the STEEM we accumulated at the launch of the blockchain, and now some of that revenue comes from selling ads. Short version, the more revenue we get, the more free software we can contribute to the Steem ecosystem.