With great power comes great responsible. I have got that flag button recently but have not used it even once because I do not sure I have a decent understanding of the instrument.
Why or when do we get this button?
When: after a user has 31 karms. Why: to mark submissions or comments as "doesn't belong here", like a very strong downvote. Too many flags from users and a submission get removed and shows as [flagged][dead] in the title.
So, any active user might have very strong downvote, but who can have not strong downvote? Like when my comments have zero or negative value.