I think the most common HN use case (mine anyway) would be to filter out topics that I don't want to see.
I use the <hide> option frequently on front page posts. It's not automatic, but it lets me manually filter rather than arbitrary automatic filter that might have more false positives than I desire.
I agree with this, it helps you to get exposed to new topics that are published by like minded people such that you're more likely to find them interesting than when just searching for completely random articles