• Every time someone posts, comments, upvotes, transfers, etc. - that all gets processed and recorded in the blockchain. The witnesses are the ones running the servers and software that validate and record all of the transactions.
• Witnesses are also elected leaders of the community. They have a responsibility to grow the Steem/Steemit community by using their skills/talents (as well as witness funds) in ways that benefit the community.
• They are the final deciders as to whether or not hard-forks occur. If there are controversial or bad changes being proposed, the witnesses can put a stop to them by refusing to pick up the new version.
• They also publish the price of Steem, which is used to determine the payout of SBD->Steem conversions, as well as the Steem marketcap - which determines the total amount of rewards available in the reward pool.
• They also determine how much interest SBD pays, and how much it costs for a site like Steemit.com to register a new account on the blockchain.

Hope that helps! If you have any other questions, let me know.