I've been using HN for a couple of years now and had no idea about this, so maybe others are in the same boat.
I originally enabled this to check why a particular comment[0] was flagged, but it turns out to be more generally interesting - especially to see the sorts of spam that HN has to put up with. I'm guessing a decent amount of that flagging is automated based on links + age of account.
[0] https://news.ycombinator.com/item?id=25049339
https://github.com/minimaxir/hacker-news-undocumented