For me it started a couple of years ago. In my non-US timezone I saw a dozen spam posts (prostitution, gambling) when browsing /new and realized not enough users see them and act (flag). The individual submissions are surely ignored by any reader and do no harm. But long-term I don't want HN to be added to any "post any link there, they don't check" growth hack lists, it would drive down quality. It's gotten better, the automatic algorithms/detection got better (I have no insight how exactly those work). My joy now is discovering automatically blocked submissions and clicking 'vouch'. Discovering good submissions, or something that might spark a good discussion, is the goal. The flagging is secondary.
(Clicked 'vouch' on https://news.ycombinator.com/item?id=29144200 while writing this, not sure why it was auto-blocked, maybe the user has several accounts)