Witnesses play a vital part in the operation of STEEM. Similar to miners.
They handle account creation (i.e. how you get the username @techguru), the price of SBD (steem dollars), are a part of the mining process (but not entirely), and much much more. Only the top 21 voted witnesses actually do anything, the others serve as backups. (thus why votes are so important)
RE: [Steemit Update] You can now vote on witnesses who are outside the TOP 50