What are Steem witnesses?

The Steem blockchain requires a set of people to create blocks and uses a consensus mechanism called delegated proof of stake, or DPOS. The community elects 'witnesses' to act as the network's block producers and governance body. There are 20 full-time witnesses, producing a block every 63-second round. A 21st position is shared by backup witnesses, who are scheduled proportionally to the amount of stake-weighted community approval they have. Witnesses are compensated with STEEM Power for each block they create.

Steemit leverages Steem because the founders of Steemit believe Steem’s decentralized text content storage and governance model makes Steem an excellent platform for supporting the long term success of its social network and digital currency tokens.

https://steemit.com/faq.html#What_are_Steem_witnesses