There are 21 blocks that are signed in a round with 3 seconds per block. 19 out of those 21 blocks are signed by the voted witnesses. 1 other block comes from a randomly selected witness. The final block is the proof of work block coming from a miner. The mining is done to cryptographically secure the database, i.e. posts, comments, votes, etc. on the blockchain.

From what I understand, the blockchain produces 2 additional STEEM every block for content creators and curators (upvoters) with the rewards distributed as per the algorithm described in the whitepaper.

Content creation and upvoting distributes STEEM to those who mainly create quality content and then followed by those who upvote the content the earliest.