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When you post a comment like this comment somewhere in the world that comment has to be stored. We run and maintain the servers that store that comment. There are a total of 20 main server distributed around the world. Having this many servers allows the steem blockchain to stay online 24/7 without any failures. IT is a type of redundancy through mass deployment.

The people like myself that run "backup-witness servers" stand ready to fill in when load is high or when a primary witness server may go off line for any given reason.

We are like the volunteer website hosting company of employees for steemit to put it in a more simple explanation.