It is also used by people getting butthurt.
I can remember that almost a year ago i made a comment somewhere that someone disagreed with and after that he flagged all my posts making them be considered spam (due to his steem power), not that those posts were of high quality i'd rather remove them and have a clean file now that i've returned to steem, but that's not possible.
I can't remember ever having flagged something myself, i have to get used to upvoting, cause on things youtube etc. i never use any of those thumbs.