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It's mostly just for bragging purposes - to demonstrate that your server is super reliable...like mine!

If a witness misses a block, it doesn't really impact the network, as the next witness will pick it up anyway. But after a certain number of blocks are missed by the same witness, that witness will be automatically disabled until they correct the problem. So it's an issue for the individual witness, not necessarily the network...unless all of the witnesses are missing all of the blocks. Then, Steem ded. In a nutshell.

You can miss some of the blocks all of the time, and all of the blocks some of the time, but you can't miss all of the blocks all of the time.

Zero missed blocks = good job.