Witnesses makes our post seem on steem blockchain, for folks to check. The communuty elects witnesses to act because the network's block producers and governance body. In excess, if there aren't any witnesses, there'll be no steem.
These are the witnesses i voted for:
RE: Show Us Your Witnesses Weekly Contest: A Concept To Drive Up Witness Voting Engagement Among Steemians (Episode #5 Result Update; Episode #6 — 6SBD Prize Pool; 3 Possible Winners)