Downvoting is a function in the steem blockchain. This function allows steem users to decrease the rewards of another user by downvoting their posts. The function can be used via the steemit, busy.org and esteem application. Downvoters can remove their downvotes if the payout period of a post hasn't passed away.
This function is useful to hide low quality posts and posts that contain illegal content. For example, child porn and similar content. Downvotes are used by some concerned users with high SP to control the spread of spam and plagiarism. But downvoting has a drawback. That is, it takes away a lot of voting power from a user.
For example, a post with 100$ payout will have to be downvoted with 100$ worth voting power to bring its payout to zero. Statistics from steem blockchain show that downvotes are very rare compared to upvotes.
Downvotes in steem make bad content invisible to search engines and that improves the search rank, allowing searchers to find good content easily. Downvotes are used to hide spam posts. A lot of spam in a site may lead to its links being shadowbanned by popular sites like reddit, facebook, twitter etc.
Posted on Utopian.io - Rewarding Open Source Contributors